2009 MacBook Pro Unexpected Shutdown

I was using Google Chrome and was on the studyblue website, a online flashcard website, and I had youtube open running a video. I was typing a not when my computer shut off on its own. I did not have any applications or files open besides google chrome. My computer was not plugged in nor was my computer running hot, and when I powered on my computer again it started as usual. I am not sure what happend I have been using Chrome for about six months without any trouble and I can't think of what else could have been the cause of this. I did have my computer in sleep mode for about 10 hours, which I usually try to avoid. I could relaly use some help. I am on Safari now and everything seems to be ok but I thought I would post a question in case this happens again.