Intel Mac :: Why Computer Won't Turn On

I was trying to download the free version of a game, and immediately after i clicked download, i started to see the computer slow down a bit. However, I just thought that it was a lag and it would fix itself when the game completely dowloaded. Afterwards, it said that there were a few errors that occured during the download, and asked me if I wanted to continue to play. I clicked yes, because I didn't think it would be much of a problem. Afterwards, the mouse was stuck on the middle of the screen, and no matter how much I wanted to click out of the window, quit Safari, or even restart my computer, the mouse would continue to go back to the one spot. I have absolutely no idea what to do to restart the computer, or quit Safari. All I have done is just sleep the computer, but that hasn't helped it at all.

Info:
iMac