Flashing file with question mark

You have to fix a serious underlying problem for it to go away. A flashing question mark can mean several things, all bad, such as:

1. the system folder is corrupted
2. the system folder is missing from the hard drive
3. the hard drive has failed or currupted
4. the hard drive has been removed from the computer

The Apple support article on the subject is here:

If your Mac starts up to a question mark - Apple Support

The option listed about using command r depends on the sub-model of your iMac, which we do not know.

If your Mac starts up to a question mark - Apple Support

Follow all of the steps, at > https://support.apple.com/en-us/HT204323