If the oven is STILL heating, but the carousel isn't turning, this problem is commonly attributed to a turntable motor that has worn out and may need to be replaced. The motor is attached to the center plastic piece (called a turntable shaft or coupler) under the middle of the glass tray. The following link explains how to access and replace one:http://www.fixya.com/support/r3555716-replacing_a_turntable_motor_on_a_microwa
This is a very simple repair, that usually is not expensive. The price range will be determined by the model and manufacturer. This repair can also be performed on an over the range model microwave oven WITHOUT uninstalling it.
You can refer to any of the following websites for comparison:
The first three sites listed have helpful exploded view diagrams to assist you with locating and properly identifying the part(s) you need.
If the model number is correct, the part number for a replacement motor is WB26X10038. You may also check the turntable shaft to make sure it is not stripped and slipping on the motor shaft. The part number for the turntable shaft is WB06X10138.
If you have any questions, please post back and let me know. I hope this helps you.