C28:4 powers up but no signal metered


I purchased a used C28:4 unit a few months ago and I've had it out a couple of times without issues.

Last weekend I fired up the system and line checked the monitors (which I power with it) and everything seemed in order. I was setting up mic stands when I heard a quiet "pop", not loud, but noticeable.

Later on when I went to soundcheck the first act, the amp did not meter any signal coming to it on any channels. At first I thought it was a routing issue with my mixer, but when I put the cable to another amp it produced sound.

Later in the day when I was telling another crew member about the amp, he said when the small pop happened he thought he smelled something electrical burning, but so subtle he couldn't be sure. 

So clearly I'm not looking for a "solution", it needs a repair. But I wondered if anyone had any insight on what I might be up against.