If the question mark is persistent, and your Mac does not start up normally:
- 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 notebook computers that have Touch ID, press and hold Touch ID.
- Use Disk Utility to repair your startup disk. Follow the steps for a Mac with an Intel processor.
- If Disk Utility found no errors or repaired all errors, reinstall macOS.
- If you still need help, please contact Apple Support.
If your Mac starts up normally except for a question mark that appears briefly during startup, just make sure that the startup disk you want to use is selected in Startup Disk settings. If that doesn't help and the question mark is temporary, check the setting again after resetting NVRAM.