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, turntable ring or coupler) under the middle of the glass tray. The following link explains how to access and replace the motor: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 may need.
The average price for a motor is about $35 to $45. The coupler is less than $10. If you have any questions, please post back with your complete MODEL NUMBER and let me know. I hope this helps you.
NOTE: Even though the carousel may not be turning, you can still use the microwave while awaiting parts. You will need to physically rotate whatever you are heating about midway through the cook time to ensure even heating. Early model microwave ovens were not equipped with a rotating carousel and this was the common routine.