Left 4 Dead 2 turns off display

[Paper]

I just finished my download of this game this morning and started it up to start playing with my brother and his friends. After like 5 minutes into the first mission that we played (it was the carnival, i don't know, i've never played this game) my display turned off, and my L4D sound kept repeating. My Teamspeak, however, was still fine, and I was still able to talk on Teamspeak. Trying to bring up the task manager, alt+tabbing, windows button, nothing worked. I rebooted my computer and tried running the game in a windowed mode, and it still occurred, this time sooner than last time. I'm not sure what to do, as my drivers are apparently updated. I'm running on a Radeon HD 7770.