Operating system not found means that your hard drive is either dead or dieing. This happened to a roommate's of mine and the drive was dead and then happened to me, but luckily mine was only phasing in and out. It would do that screen most of the time, but every once and a while I could get it to boot up. If you are lucky enough to get it to boot up, transfer all of your important crap off of it, because you need to put a new drive in. It sucks.

Also, if yours is already dead, I hope there wasn't anything you needed on there, because data recovery is wicked expensive.

Sorry I couldn't be any better help, maybe someone else knows more.