• If the question mark shows briefly before your Mac starts up regularly, follow these steps:
• NVRAM should be reset.
• In the Startup Disk settings, make sure your preferred startup disc is selected.
• If the question mark continues, try the following: For up to 10 seconds, press and hold the power button on your Mac until it goes off.
• To start up from macOS Recovery, turn on your Mac and immediately press and hold Command () and R.
• Use Disk Utility to repair your starting disc while in macOS Recovery.
• Reinstall macOS if Disk Utility discovered no issues or corrected all faults.
• If you want further assistance, please contact Apple Support.

Read More: What to do if you see a blue screen at startup