Today i went to wake up my iMac from sleep and it wouldn't start. Eventually i got mad and manually restarted it. It was working fine till i used expose and it suddenly froze with like a checkerboard pattern on the screen. I let it sit for awhile and nothing happened so i restarted it again and at start it has this checkerboard pattern. ( its hard to describe, its like multiple color fuzz and the regular background squares). The iMac wont start it just sits there with the display all messed up. I am running a 20 inch Intel imac. Please help me.