The most likely cause is a power glitch that has zapped your mobo.
The PSU itself may have been the cause or there was a mains spike.
If it was RAM, the PC would beep during boot and put up a message. If booting can't get that far, then the mobo cn't process the boot instructions. if the PC shuts down, the PSU isn't supplying the right power.
I think that's about it. Could be wrong, but it's the olkd story you've told.
Anyone disagree?
Suspishio
Posting Virtuoso
1,730 posts since Aug 2007
Reputation Points: 47
Solved Threads: 145