Screen gone black? PLEASE HELP ME :S

I bought a 21.5" iMac today with OS Lion, and I've been playing around with it for a few hours. However, my screen has just suddenly gone black. I read the manual and I thought that it had just gone to sleep, but it seems to be completely frozen. I've tried unplugging it and restarting it, but I've had no joy. What do you suggest I do? Thanks :3