My compaq shuts off randomly, but only when I'm not using it...?

[Levmeister]

I only got it a couple of months ago, it's a compaq presario notebook and it is really good. However, overnight, while the lid is closed and the computer is asleep, it shuts off. It doesn't happen every night, but maybe every 2 or 3 nights, i get home from school, open the lid, and it's off. When I turn it back on, it says 'resuming windows' and even if i had a game up it goes back to the exact point it was at. I don't seem to be having any memory loss or anything, but its really annoying, so if anyone can help...thx...