support.apple.com/mac/startup black screen w ques
Black screen .. won’t load anything w grey question mark box blinking
iMac 27″, macOS 12.7
Question marked as
That means the computer cannot find the startup volume. Please refer to If your Mac starts up to a question mark - Apple Support
Depending on the age of the computer, that may mean the start-up drive has failed. If it has failed then it becomes a question of "Do I take it to an Apple Authorized Service Provider for replacement?" or "Is it time for me to shop for a replacement computer?"
My advice is, if it's a 2017 or earlier machine and the startup drive has failed, then it's time to replace the computer.