Common for the reverse band to break in these transmissions. Need to have it scanned for possible codes. There is a very simple test that can be done just by driving the vehicle but every time i tell folks what to do they ALWAYS get it wrong! So just let a MECHANIC test drive it, can tell rather quickly if it is a mechanical or electrical problem, just by driving it and knowing how to test for manual low! Very simple and bulletproff!
Good luck with it.