My Mac starts up, but the screen is garbled

With a 15in MacBook Pro (2011-2013), graphics processor failure is a known problem. Apple may repair later models (see bit.ly/mbpvidiss) but otherwise workarounds (such as bit.ly/gpuwkrnd) are poor/impractical. For other models, try Apple Diagnostics (see page 25).

If your Mac starts up with a flashing question mark icon, it means it can’t find a drive with a macOS installation. Let it try: if it does eventually start up, go to  > System Preferences > Startup Disk and select the correct drive. If not, try macOS Recovery (see page 25).

If your Mac starts up with the screen overlaid with ugly text, it’s a kernel panic. Restarting again may work. If not, restart in safe mode (see page 24) and install any software updates. No? Unplug all peripherals, then try restarting with them one by one.

If the screen stays grey, be patient: macOS may be doing housekeeping. Still nothing? Try safe mode with peripherals disconnected, as above. Reset the SMC. No? If possible, remove any extra memory you’ve added (with power disconnected) and try without it.

An app is playing up

If an app behaves oddly, quit (or force-quit – see page 21) and reopen it, restart your Mac and check for updates in the Mac App Store or from the app’s menus. No joy? The app’s preferences may be corrupted. Quit the app, switch to Finder and go to ~/Library/ Preferences (see ‘Where’s my Library folder?’, page 22) and drag anything obviously named after that app to the Trash. With luck the app will now work, but you may have to set its options to your liking.