I honestly don't know.
It happened in the heat of the day, though I didn't think it was that hot in the house. I had been experimenting with different fans and had just put some Cooler Master monster heat pipe cpu coolers in. I thought the northbridge (e7500) was running a bit warm, but still within limits. I have a side fan which pretty much blows directly on it.
My wife was running it - she said it didn't even blue screen, it just spontaneously shut off. She thought the power had gone out.
At first, nothing. NOTHING. I reset the PS, and finally the system turned on - but no pwr LED, and the pwr switch has no effect, it just turns on when you connect power - and the monitor never turns on.
No post, nothing. All voltages present, and subsystems appear to be working - HDDs run up, NIC connection talks to the router.
I'm not sure what it is, but I know enough to know it looks bad. So, I am going to re-load my old machine (an old dual P3) until I can set aside a few afternoons to track down just what is wrong with it, and decide if it is repairable.
*sigh*