Simmons DA200S drum amp will not produce sound.

I plugged a cord from a monitor amp into my powered speaker. I know, duh! It made a noise which sounded like it was shorting out, and then no sound. When I turn it on I can hear a slight hum, but no sound comes out of amp when I plug in my mp3 player.

The fuse on the back of the amp is not blown. I have opened it up but do not see anything obvious on the electronic boards indicating an issue. Which component would most likely be damaged in this scenario?

Thank you in advance for any help you may be able to provide.