Buddy, you need to first see if the dropped channel is from all sources. That means it’s most likely a cable between the amp and preamp, as someone else here has said. If not, I would pull everything apart and start over. When you put it back together, put identifying tags on all your cables.
Otherwise, get a service guy from an audio store to help you.
@unreceivedogma your dogma has been received and is in the queue.