did you check the speaker output jack on the amp and cab to see if they look loose ? did you cehck that speaker cab with another amp ? just basic rules
I used the same speaker cab last night at a gig, and today and that was fine just using my RP300 pedal into my crate SPA200 power amp. I tried both speaker jacks on the amp with said cab. The damn thing just worked the other night.
Last year in july it was F'd up and I didnt get ANY sound out of it. the standby light wouldnt even come on, once I took it to be looked at it ended up being a $0.37 resistor that THANKFULLY did the job that resistor was supposed to do, or it would have blown the whole amp. Cost me $50 for that transistor to be installed but she was screamin after. She's probably going to take a trip to the Doc again.
Time to start saving the pennies to get a back up tube amp.
Zander I dont know everything your runnin but disconnect everything from the effects loops and hook up the speaker cab to the speaker out on the amp. If you have sound that way the amp is workin. From there check the effects loops...Guv