Doesn't boot up, flashing strait lines on screen.

I got this computer for my son when he started college almost 3 years ago. About a year ago he came home for visit, was cooking from recipe on computer, had it open on kitchen counter, managed to spill about 3 lbs of sugar on keyboard. I was there, turned computer over, vaccumed it out, it worked ok and he continued using it at school. However, he took it to Apple Store recently because optical drive didn't work and earphone jack didn't work. Apple Store folks at first said no problemo, it was still under Apple care, took it apart, but when we went to pick it up we were told they couldn't fix it under warranty after all, it'd cost us $750 for them to repair. Thing is, the computer worked before the Apple folks took it apart, now it won't boot up, the CD drive makes noises for a while when it's first turned on, then there are flashing colored horizontal lines on the screen. Can't find this phenomenon in troubleshooting guide, anybody have an idea what it might be? I followed instructions for replacing I think it was the fan up to step 12, took keyboard off, found no sugar inside, put it back together, no change in performance.

Dan