2009 mac desktop won't turn on

That’s a firmware problem. It is attempting to restore the firmware. Do you see a progress bar on the screen? If so, wait… it can take a while. If it fails or otherwise can’t do it, it’s time for a new machine. That one is not worth repairing.

If you’re not seeing the progress bar at all, it’s probably toast. If you didn’t have your data backed up, probably the only option will be to physically remove the hard drive so that it can be connected to another machine to try and extract the data.

See if the machine can boot into Target Disk Mode. If it can then you'll be able to connect it to another Mac and copy all of the user data from the old Mac to a new one.