Have you made sure that the signal path hasn't been interupted. Check to be sure that the wires going from the preamp section to the reverb box (usually at the bottom of the speaker enclosure). Also, since the Twin Reverb is an all-tube amp, you may have a bad tube that controls the reverb mix. Any good guitar shop can check the tubes. I do suggest you contact the folks at Fender or an authorized service center. I know Fender has a web site but I can't recall the web address. A fast trip to www.excite.com
will take care of that.
I know Im about a month late on this, but you might try taking about the reverb box. I've done this on my Yamaha and my polytone amps, and found that the actual reverb spring was either unhooked, or broken. It was pretty easy, and Im not very technical.