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.
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.)
Follow the steps in this article: How to repair a Mac disk with Disk Utility. It describes how to start up from macOS Recovery, then use Disk Utility to repair your startup disk. It also describes what to do if Disk Utility can't find or can’t repair your startup disk. If Disk Utility found no errors or repaired all errors, restart your Mac: choose Apple menu > Restart.
If the question mark returns after restarting, press and hold the power button to turn off your Mac again. Then follow the steps to reinstall macOS from macOS Recovery.
If the question mark isn't persistent, but appears only briefly before your Mac starts up, make sure that your startup disk is selected in Startup Disk settings. If that doesn't help, reset NVRAM and check the setting again.
