If your Mac starts up to a question mark
A folder with a flashing question mark means that your computer's startup disk is no longer available or doesn't contain a working Mac operating system.
Turn off your Mac
Press and hold the power button on your Mac for up to 10 seconds until your Mac turns off.
Every Mac has a power button. On laptop computers that have Touch ID, press and hold Touch ID.
Start up from macOS Recovery
To start up from macOS Recovery, press the power button to turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R.
When in macOS Recovery, your Mac shows a list of utilities, including Disk Utility:
Use Disk Utility to repair your startup disk
Select Disk Utility in macOS Recovery, then click Continue and follow the steps to repair your startup disk with Disk Utility. These steps also describe what to do if Disk Utility can’t repair your startup disk, or Disk Utility doesn’t see your startup disk.
When complete, restart your Mac: choose Apple menu Restart.
Reinstall macOS, if necessary
If the question mark returns after restarting your Mac, repeat the previous steps to turn off your Mac and start up again from macOS Recovery. Then follow the steps to reinstall macOS.
If you need help
Find out what to do if you can’t start up from macOS Recovery.
If you still need help, please contact Apple Support.