727 no reverse

Pull the pan and see if the band strut fell out. If it was not installed properly it can pop out. Adjust band to 72 in/lb and back off 1.5-2 turns. Also check the servo for the rear band for cracks or damage to the seal.
You will have to pull the pivot pin to get the servo out. It will require removal of the tailshaft but can be done in the car. I cut the tab off and make a bracket that retains the pin so it can be pulled easy without removing the tail shaft.