Bill,
If your motor has a round electric (magnetic) brake mechanism on the end of the shaft, made of metal with two wires going into it, try taking it off and try the slide again. Sometimes there will be dirt or surface corrosion in it that can lock the motor shaft. It can be cleaned with spray brake cleaner and the shaft rotated with a screwdriver in the slot until it is free and then reassemble. This is a bit of a long shot but those motors have been pretty reliable. You might check voltage going to the motor to be sure it is powered properly when the slide switch is pressed.
Later Ed