Any idea on why my computer just shut off on me...?

I was on it last night on AIM typing a msg and BOOM! It completely shut off. Just before, I took it off its charger, but the battery was at full strength. My only other thought of what happened is a virus?? I have had my computer on for close to 2 weeks. Maybe it just needs rest? Still, I do not understand why this happened.

It is a Compaq laptop I got 3 years ago