Given the age of the computer, try replacing the CMOS battery on the motherboard. (You will have to reenter your personal settings) See if that resolves it.
Also try the back up bios switch and toggle it, try rebooting.

If if flickers back to life, try reflashing BIOS, sometimes you get "bit rot".

My son's mobo recently gave up the ghost as in no POST, though with some error codes, with known good CPU and RAM. Then we got no error codes or POST...then it is time for a new motherboard.

Cheers