What happens is that a mechanical system under the turntable pushes on a micro switch to operate the motor. Clearly either the mechanical system has moved out of position or the switch has. You take the bottom off the turntable and see what happens when you move the arm.
First check if the tape is moving during playback. If it is not there could be something wrong with the mechanism, such as a drive belt being off. If it is moving. Then it could be that a tiny leaf switch has become broken or bent cutting the sound off. Other issues include a faulty switch that controls the switching from the radio to tape. Could be just dirt in the switch. Or the pre-amp failure of the cassette player.
