More serious problems
WHAT DOES A FLASHING QUESTION MARK ON STARTUP MEAN?
Your Mac can’t find a drive to boot macOS from. The first thing to do is wait, because occasionally a minor problem causes this to happen as a one-off. If the Mac eventually starts up, go to System Preferences > Startup Disk and select your startup drive. If not, try Recovery (see ‘How do I use Recovery to get my Mac working again’, below right).
