If the question mark only appears momentarily before your Mac starts up normally:
If the question mark persists, follow these steps:
- Press and hold the power button on your Mac for up to 10 seconds until your Mac turns off.
- Turn on your Mac and press and hold Command (⌘)-R immediately to start up from macOS Recovery.
- While in macOS Recovery, use Disk Utility to repair your startup disk.
- If Disk Utility didn't find any errors or has repaired all of the errors, reinstall macOS.
- If you still need help, please contact Apple Support.