Underneath there will be a belt driven by a motor - very probably either the motor has failed or the belt has broken.
For safety reasons users are not encourages to mess around inside microwaves the expectation being that a trained specialist will attend to it.
If you do decide to ignore convention and have a go yourself then take great care - lethal voltages lurk inside even with the power cord pulled out!
Good luck...
Peccavi
July 2013