My God...could be damn near anything...
What you have to decide is if it is worth fixing. If it uses a STK-type output module (and the failure mode you described sounds familiar for these devices), then it may well cost more than it is worth to fix.
However, the worth of most audio gear lies somewhere beyond it's eBay resale value. I have a few small amps here that I have invested many many times more money in than what it would ever sell for on eBay or anywhere else. Sometimes it just works out that way.
The first step here (if you harbor any thought of getting it running again) is to get a copy of the service manual. $20 or so will tell you a lot about the amp, including what it uses for output. If it does use an amp module for output, then you need to check to see if one can be found and how much it will cost. If it uses descrete output devices, then the repair is going to be a lot easier and probably cheaper.