If your Mac starts up to a flashing question mark, but then doesn't continue starting up, try these steps.
1. Turn off your Mac by pressing and holding the power button for a few seconds.
2.Press the power button once to turn your Mac back on. Immediately press and hold the Command and R keys on your keyboard to start your Mac from macOS Recovery. Keep these two keys held down until you see an Apple logo or globe.
3. If prompted, select a Wi-Fi network to connect to the Internet as part of startup.
4. After the macOS Utilities window appears, choose Startup Disk from the Apple menu.
5. Select your startup disk, then click Restart.
If you don't see your startup disk in the Startup Disk window, close the window and try the next sections of this article.
Credit to Apple Community Specialist.