Funny.... I had the same thing happen to mine not to long ago.... put something in to microwave, and POP! scared the bejeebers outta me! I tried a few things, unplugging it and plugging it back in, different buttons, etc, but nothing worked. It was an older microwave, so rather than mess with trying to fix, we bought a new one. About $100 at Sears.
Hate to say it, but you might have to buy a new one! :?