When you took it apart, you flexed it.
There are a couple patch jacks on the front - Preamp Out and Power Amp In. Poke a plug in and out of the power amp jack a couple times. Did that wake it up? If so, spray some cleaner in that jack and push a plug in and out a few more times.
And if no help, plug your signal into the power amp jack instead of the regular input. Sound OK? Or does it still sound crappy?