One thing that happens that is a cheap fix. It has happened to me a couple times on old Macintosh motherboards where there is a watch-size battery placed to keep the time and other memory settings constant when the power is off for long periods of time. Over the years this little battery dies and that renders motherboard non-bootable. Possibly on old PCs there is a similar little battery. It's quick and easy and cheap to check. No guarantees of course.

GOOD LUCK