When bootsequence detects a error in it's cmos, it resets to standard settings. This could happen due a critical HD writing error, something made the HD do crazy things. Or a virus designed to attack the cmos.

Scan for virusses, did the pc at that time suffer any physical disturbances? A fluke in the powersupply, shaking? I've experienced similar problems where my laptop refused to boot when been transported on rough surfaces, in the back of the car. Due to the vibrations.


It's one of these days...