I have a desktop system that ran fine for months, but now it doesn't even hit BIOS most of the time.
I've replaced a battery on the motherboard (CR2032) and the power supply, and the behavior hasn't changed.
If I disconnect the box from power for a long time (several hours) and then plug in the system, sometimes I'll make it past BIOS. One time I made it to the login screen and way able to login. After a few minutes, the OS froze and I had to hard reboot. Sometimes the boot sequence will make it as far as a screen that says "hit delete to setup BIOS", but after a few reboots, I wont even hit that screen anymore, and nothing will be rendered out to screen.
additional details: the motherboard and video card are older and given to me by a friend. All of the hardware is out of warranty.
please let me know if there are any details you would like to know.