Hold down the Command and Option keys together and press the Escape (esc) key – it’s at top left of your keyboard. You’ll get the Force Quit dialogue.
Click on the troublesome app listed (it will be clear which one it is, it’s marked ‘not responding’) then click the Force Quit button.
Unsaved changes may be lost, but you should be able to reboot the app and keep working.
Application frozen and can’t use mouse/trackpad/keyboard
Holding in the Mac’s power button for six seconds or longer forces any Mac to restart. This is a bit radical, but effective. Restarting flushes the RAM (a plain restart can solve many problems in itself, as it forces the system to reset various parameters).
CD, DVD stuck in the drive
For older Macs that still have optical drives, and for external optical drives – restart, and as soon as the Mac restarts, hold down the mouse button or trackpad-click. Keep it held down, and the disc should eject. Once it does, you can let go.
Disk Utility and Repair Permissions/First Aid — If your Mac slows down and things keep going wrong, this can fix 90% of problems. Apple placed a utility on your Mac which resets the underlying Unix file structure, which can get corrupted or bogged down by unnecessary directory code.
For example, you have file X in your documents’ folder. Unix sees the direction to find this file as /(your user name)/documents/X. If you move it into a different folder, Unix lists the new path as /(your username) /documents/(different folder)/X. The forward-slash represents a folder, or (in Unix-speak) a Directory. But the previous unused path is remembered – loads of these can seriously slow down your Mac. Also, connections that Finder needs to find and link files can get corrupted or confused.
Solution — In Finder (ie that it says Finder next to your Apple menu at the very top left of your screen) open your *Utilities’ folder from the Go menu (it’s in your Applications’ folder) then open Apple’s utility app called Disk Utility.
The layout and operation of Disk Utility changed under El Capitan. Open Disk Utility up, select your internal Hard Drive on the left (it’s usually called Mac HD unless you’ve renamed it) and click First Aid at the top left. You will be promoted ‘Would you like to run First Aid on Mac HD …’ Yes you would. Click Run and it does much of what the older version (see below) did. If you expand Show Details you can see what’s being operated on.
You can select any plugged-in external Hard Drive or even USB stick on the left and run First Aid over them,
If you have an older version of Mac OS, open the Disk Utility as above. Now, make sure the First Aid tab is selected, click once on the icon for your HD over on the left and click Repair Permissions, which finds all the places where all the files are, deletes all the incorrect paths and writes fresh new paths. Result – a faster, happier Mac.
*In Unix speak, your path is
‘Macintosh HD/Applications/Utilities/Disk Utility’.
OS X’s Safe Mode Boot
From Lion OS 10.8 on (so also Mountain Lion 10.9 and on Mavericks 10.10) users can use an interim step that verifies the directory structure of your hard drive at startup.
To start up in Safe Mode, shut your Mac down (choose Shut Down from the Apple Menu). When it’s off, start up your Mac and immediately hold the Shift key on your keyboard down until you see a grey screen with an Apple icon and a progress bar beneath it. Safe Mode takes a while, so don’t be alarmed that you don’t see the desktop right away.
The progress bar indicates the operating system is verifying the directory structure of your startup volume. It will repair it, if necessary. It will also delete some of the startup caches that may also be preventing your Mac from starting successfully (these will be recreated fresh).
Once the desktop appears, you can access and run Disk Utility’s First Aid (Repair Permissions) tool just as you normally would, as above. When the repair is finished, restart your Mac normally.
Not all applications and OS X features will work when you are booted into Safe Mode. Just restart afterwards for normal operation – you use this startup mode only for troubleshooting and not for running day-to-day applications.
This is OS X’s answer to not supplying a startup disc, since from Lion the Mac OS has been a digital download.
You may notice, in Applications>Disk Utility>First Aid, there’s a Repair Disk option. It’s greyed out and unusable on the hard drive your system is installed on. As an analogy, if you’re a heart surgeon you might be able to fix a damaged finger, but you can’t operate on your own heart.
But If you have an external hard drive plugged in, you can select that on the left and Repair Disc will be active. Use it to repair any external drives, for example Time Machine backup drives, extra hard drives and even thumb/USB drives, every now and again to make sure all is good.
(I never bother with Verify. It takes as long as Repair, except all it does is tell you yes, you have problems – or not – without fixing them.)
To fully repair your internal System Disk
You have two options if Repair Permissions doesn’t work and/or your internal hard drive is still causing problems:
For OS 10.6x (Snow Leopard) and versions of OS X before that, put in your original Mac OS install DVD (Lion doesn’t come on an install DVD – see below), restart and immediately down the C key on your keyboard. Keep the key down until you can hear your Mac laboriously booting from the DVD. The system is booting from the OS on the DVD instead of your Mac (so it’s slow).
In the installer that eventually appears, select ‘Use English for the main language.’ When this option appears, click the arrow button.
Now select ‘Disk Utility’ from the Utilities menu – in the First Aid tab, select your Macintosh HD on the left and you’ll see Repair Disk is an option using this method. Press Repair Disk, hope and be patient.
Please note — if this tells you it can’t fix it, you need to take your Mac to a technician, sorry. So I hope you have a backup…
For OS 10.7x (Lion, Mountain Lion, Mavericks, Yosemite, El Capitan …)
OS X includes a built in set of utilities in a Recovery HD partition. To access these, restart your Mac and hold down the Command key and the R key (Command-R), and keep holding them until the Apple icon appears, indicating that your Mac is starting up. After the Recovery HD is finished starting up, you should see a desktop with a Mac OS X menu bar and a “Mac OS X Utilities” application window. Note: If you see a login window or your own desktop and icons, it is possible that you didn’t hold Command-R early enough. Restart and try again. Apple has more info here.
Apple Hardware Test
As Apple’s support site says, you can also use the Apple Hardware Test to see if anything physical is wrong with your hardware.
Disconnect all external devices except the keyboard, mouse, display, and speakers. If you have an Ethernet cable or external DVD drive, disconnect them.
Restart your computer, holding down the D key while the computer restarts.
After your computer restarts, you should see the Apple Hardware Test chooser screen. If you don’t, Apple Hardware Test may not be available on your computer. You may be able to start Apple Hardware Test from the Internet. Reconnect your computer to the network, and then restart your computer while holding down both the Option and D keys.
When the Apple Hardware Test chooser screen appears, select the language you want to use, and then press the Return key or click the right arrow button.
When the Apple Hardware Test main screen appears (after about 45 seconds), follow the onscreen instructions.
TechTool Pro — Presuming (and hoping) you back your Mac up regularly, TechTool Pro (it’s not cheap) is the go-to tool for Mac evaluation and repairs. Most pros use it, although they may also have other tools. It goes much further than Apple’s Disk Utility to run a variety of tests.
Unix Fix Disk
This can be a life saver on all versions of OS X. It is brilliant if you can’t find your original OS DVD, or you have Lion installed (which doesn’t come with a System Disk), or you are on a plane or something.
Restart, and hold down the Command and S keys together. Keep them held down until the Mac starts up with a black screen with white text appearing.
You are now running your Mac as a Unix box.
When no more text appears, type ‘fsck -fy’ without the quotes. That’s Eff Ess Cee Kay space hyphen Eff Wy. It may look rude, but it’s the Unix Fix Disc command.
Press Enter, and be patient as Unix checks and reorders your file directories and finds disk problems. It also, 99% of the time, fixes everything.
If all appears OK, after word ‘root’ type ‘reboot’ and press the Enter key on your keyboard. Your Mac will happily restart in OS X, everything fixed and working faster.
But if, when it’s finished, if problems are listed, run fsck -fy again as sometimes this works.
But it lists a problem the second time, it probably can’t fix it, so it’s time to go and see a technician.
Mac won’t even start up? Try resetting the SMC
Try resetting the System Management Controller (SMC):
1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source if you’re using a laptop, if it’s not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.
Here’s the full story on how to reset the System Management Controller:
Apple Services Status Page
If you are having trouble with an Apple online service (iCloud, Mail, iTunes Store, Mac App Store etc), before you panic, check Apple’s Online Services Status Page as sometimes the outage comes from there.
iPhone/iPad error codes: Troubleshooting iTunes error codes
If you have you encountered a weird error code when you connect your iPad or iPhone to your Mac (they may pop up when you are updating your iOS device software or backing up your iPad or iPhone to your Mac) at TUAW you will find a list of these errors, an explanation of what they mean and some instructions to help you resolve them.