If you've checked all your tubes and fuses, as well as verified that your guitar and cab (or reactive load) are working, next step is to call a tech, as you could have a damaged component somewhere on the board that isn't as easily tracked down or replaced as tubes or fuses.
Something else to be aware of is that I recently had a similar thing happen to a 5150 III. No volume, all fuses and tubes tested good. I took it to my local tech and he was able to track down the issue. Turned out there was a small metal shaving inside the gain pot positioned in just such a way that it was shorting the entire signal to ground.
Just one more thing to consider.