Think about it, if at some point in the rotation of the reverb control you have straight signal, then you know that the straight preamp is all working. If at some other point in the rotation you have reverb sound, then you know that the reverb circuit is working.
Then if at some point in the rotation the signal completely drops out, then the problem could be a bad or dirty pot, or it could be that somebody has mis-wired the pot or replaced it with the wrong pot.
Does the wiring to the pot look different? Does the pot look like the other pots in the amp?