There is a small turntable motor underneath the floor of the microwave, this can sieze up. The drive unit from motor shaft to the turntable (its called a coupling) can crack and allow the motor to turn without it moving. Remove the turntable plate and put a cup of water in the microwave, select lowest power 10% for 30 seconds and look thro the door to see if the drive coupling is turning. If not, investigate the coupling and motor.
ukanfixit, March 2012
