One day mg100hdfx the next nothing...help!?!

[lespauly]

So I bought a marshall mg100hdfx from craigs list and for a few months it was fun never had a half stack before and I was enjoying this one. For a while though I was just using the cab and an ext. speaker cab for my tube combos, then I decided to hook the original head to the cab. When I turned it on it had a crackling noise at first then it played like normal so I thought nothing of it. Few days later I turned the amp on and again the crackling but this time almost all the volume was gone, for example with the gain and the master volume cranked I could hear the guitar through the 4x12 but only loud enough to hear it over the natural sound of the guitar. I have tried the connecting the fx send and return together that didn't work and the fuse is not blown. I have very limited knowledge of electronics but I am will to try anything...so if anyone has any ideas they would be greatly appreciated.