Why does my computer freeze?

So it was a normal day right? I was playing LoL and it wasn't working. So I played an hour or so of Modernwarfare 2. After that I decided to restart my computer becuase my McAfee has been telling me to for some updates for the past 3 days or so. Although I held the power on...When it turned on, it froze and the windows logo of the green,yellow.red. and blue thing. So i force restarted again. This time halfway when it was loading at the Dell screen it just froze and my Num Lock turned off. I coudln't press f12 or f2 or anything.