I would becareful doing that, I bet it just resets the BIOS on the motherboard hence the reason why the power usage is dropped. Should be fine though.

You'll probably find the hard disk has failed. Did you ever defrag it or do any hard disk 'house keeping'?

Have you heard any strange noises coming from it?
Did the laptop seem to slow down now and then, making repetative cycles?
Have you received any viruses or suspect emails?

It could be anything, laptops are fickle things and are prone to the power pack (I know you said that wasn't the reason) and other items just stopping.

My theory is that the processor has shut down. Do you leave it on for prolong periods of time? I know that most modern laptops from a year or so ago, especially in the case of the celeron chips, used desktop processors instead of their mobile equivalent. This leads to a much shorter processor life but is much cheaper and slightly faster than using the mobile chips made by intel or AMD. It is possible that, with prolonged periods of use, that the chip has finally given up.

Of course, I could be completely wrong and the BIOS is screwed.