Windows 7 Suddenly Goes Into Sleep Mode - Unknown Error

I cannot provide a BSOD error code/info because there was none generated; there was no BSOD. What happened was that I had a few applications open. MSN, Firefox, Opera, and Songbird are what I normally have open, and they never caused a problem before. Then I had Google Chrome open, which I do at times with no side effects, and was playing Angry Birds. The link I used to play is this one: Angry Birds Chrome, then after about an hour playing, the computer suddenly went into sleep mode. It refused to wake up. I had to force reboot it. It was able to load up just fine after that. I looked in the event logs and found nothing to determine the cause of the sudden sleep mode.

The only thing I did different is playing Angry Birds. I don't know if it being in HD had anything to do with it, or if Google Chrome just experienced a glitch that affected the entire system somehow, though I don't know the odds of that. Another thought I had was the problem was due to a storm outside. It was raining and thundering outside. A friend of mine suggested the issue may have been due to a partial power loss that was enough to knock the computer into sleep mode, but not strong enough to cause a full on power outage.