Hi Irv.
Probably too obvious, but my initial thought is maybe the switch next to the winding lever has inadvertently been knocked off of the "Single" position?
Slightly different design on my c330... it's a proper two position switch, but from what I can see, on the c33 it's similar to the rear door catch, which is prone to spinning out of position.
Looking through the old "focal press" Mamiya C book, there's no trouble-shooter as such... although it does say that the shutter has to be cocked manually on the lens when the multi-exposure setting is selected, which sounds like the symptoms you're describing.