Taken directly from the following Apple Support article: About the screens you see when your Mac starts up
"If you see a folder with a question mark appear instead of the Apple logo, it means your Mac couldn't find a local or network-based startup disk. This can happen if the disk selected in the Startup Disk pane of System Preferences isn't available. Wait a few seconds to see if your Mac is able to locate the startup disk you specified.
If a question mark appears after you install a software update, select your startup disk again using macOS Recovery."