If the rest of the oven works fine, then this indicates that you might have a problem with the motor that turns the turntable.
If you try to turn the turntable manually (gently), does it turn freely or is there resistance? If it turns freely, the mechanical mechanism between the turntable and motor may be broken and the motor may be ok.
The oven's
microprocessor also controlls the motor. There could be a problem there.
You'll need to have it looked at by a service center. Although its not that difficult to determine which of the reasons I mentioned above is the cause, it can be however, dangerous to operate a microwave oven with the cover off and with high voltage circuits exposed.
Please let me know if I can be of further assistance to you.
Good luck!
- Jim