Check the spring reverb tank for foam. The guy who sold me my Rectoverb told me that the when he got it, the reverb didn't work because the foam placed in the reverb tank to prevent the springs from slapping around during shipping was not removed. Naturally, this also prevented the reverb from working. If you've got a combo, the tank should be strapped to the bottom of the cab in a leatherette case. Worst part is taking the back off the cab... so the whole thing's a pretty easy fix. If you don't find anything inside the tank, do the obvious checks of making sure it's hooked up properly, and if all else fails, take it to an amp tech.
