Over the years, I've been given several PC's that were apparently DEAD.
Once on the bench, I pulled the CMOS battery(s) and checked them with a digital volt meter.
In every case the battery was effectively Dead (checked out at less than 3 vdc.)
After letting the PC set, totally UN-Powered, for a half hour or so, I put in a new CMOS battery, plugged
in the power cord and the PC would boot up normally. Then after resetting the time, date, etc, in
the bios, the PC would boot up and run normally as if nothing had ever happened.
If this only happened once, I'd probably not even mention it, but since I've seen it happen
several times, it's at least worth a mention.
If you had a PSU tester, I'd suggest testing the PSU for a missing voltage.
My own PC failed to boot recently and I pulled out the ram sticks and cleaned them and then the PC booted up normally.
So there are several things that can prevent a PC from booting up.
Good Luck,
The Shadow
