A microwave oven can be a dangerous small appliance to repair. Microwaves are high-voltage appliances that emit radiation; therefore, you must use extreme caution even when you are just trying to diagnose a problem. Although most problems with a microwave turntable are easy do-it-yourself fixes -- for instance, you'll be able to observe whether food particles are obstructing the glass carousel cooking plate or tray -- mechanical problems affecting the operation of the oven should be left to a professional who has experience repairing electrical appliances.
Turn off power to the unit, and unplug the microwave. You do not want electricity running to the appliance while you are working.
Remove the glass plate or tray from the microwave oven. Check to see if spilled food particles are caught in the roller wheel or drive bushing that supports the glass plate.
Clean the inside of the microwave, particularly the area below the glass plate and the plastic turntable ring and roller wheels. Food particles may be obstructing the motion, preventing the turntable from rotating. Gently wash with warm, soapy water to remove any cooked-on spills.
Place the turntable ring back in the circular groove in the bottom of the microwave oven. This groove keeps the turntable ring in place.
Reposition the glass plate so it lines up properly. Grooves located in the center of the bottom of the glass plate should fit onto the drive bushing, a plastic mechanism located in the bottom of the microwave's interior. The tray will not turn if it is not positioned properly on the drive bushing. Sometimes this part becomes worn or cracks.
Test your microwave to see if the turntable rotates once you have examined and cleaned these components. Do not run the microwave without anything in it, though: Boil a cup of water when you test the appliance.
Contact a certified appliance repair technician if you suspect a mechanical problem. Do not try to repair or replace powered parts in the oven yourself, as you could receive a serious electric shock from the microwave even if it is unplugged.