Hmm.. Yea it was a full system hault. I had to do what the above poster said, pressing the power button till it powered off. This only happned once so far. Once the computer came back on i played again, for over an hour and all seemed good.
Actually, since i'm new to macs, i've noticed something about them when they lock up (Only happened once on mine, and once on a friends older CD mbp), but when the apple logo pops up and starts to load up os x, how come it takes so much longer to load up compared to when the system was shut down normally? Does it run a bunch of tests during the startup if it crashs?