Maybe, the battery that provides the current for remembering the CMOS settings (and the clock) when the PC is turned off, is empty.
While the cmos battery does save the bios settings, it is not required for the computer to boot properly in which is the case. And you can easily test the cmos battery by touching it to your tongue [the two metal contacts not just a random part of the battery] (fyi make sure you wipe it off before doing so...who knows what's been sucked into your computer and landed on the battery since it has been installed).
Sounds like a power supply issue or the motherboard is gone. If you happen to have an extra power suppy laying around try swapping it with the known working one and see if you can get into bios without any issues. If you can then its the PSU, if most likely is the motherboard.
Could also try vacuuming the whole motherboard off as a last resort, to clean it of any dust that could have helped form a short, and then removing the cmos battery and unplugging the PSU from the motherboard and letting the motherboard capacitors drain off all their juice (around 2-8 hours, overnight is always best). And then reinstalling the cmos battery and then reconnecting the PSU and try booting the machine again.
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