my Mac mini desktop starts with a black screen and ?

Command-9 doesn't do anything.

If you are talking a flashing ? floppy disk, that usually means it can't find the startup system on the disk.

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

If you have data to recover from the disk, seek a professional Mac OS data recovery specialist.

You could try some data recovery software that provides its own startup disk/flash drive, but depending on the cause of it becoming unreadable, you could make it unable to see the data with further evaluation.

Recovering data from Macs - Apple Community

If you have data on the boot drive that you need to recover you might be able to recover it with another Mac by connecting the Mini to it via Target Disk Mode. Try booting your Mini into Target Disk Mode and if it does you can connect the two Macs by the appropriate cable and reboot both, the Mini into Target Mode. The Mini will show up on the other Mac as an external drive.