Odd occurance with Power Mac

I was watching a video for the 3,000,000th post contest, and was about to close the window, then all of the sudden my second LCD screen went dark and about 1 or 2 seconds later the mouse disappears and the computer freezes. I had to do a hard shutdown via the power button. Any idea what that could have been? Everything seems to be fine now, but it just seemed like an odd and random thing.