Diagnosis
The drive motor underneath the cavity has failed or the coupling between the motor and the glass tray is broken.
How to Fix It
Remove the glass tray and roller ring. Inspect the plastic drive coupler (a small, white, three-pronged hub) in the center of the cavity floor. Ensure it is not cracked and is seated properly on the motor shaft. Reinstall the roller ring and tray, ensuring the tray's notch aligns with the coupler.
If That Didn't Work
The turntable motor has likely burned out. You can confirm by listening for a faint hum when the microwave runs; no sound indicates a dead motor. Motor replacement is required.
Heads Up
An improperly seated or warped glass tray will also inhibit spinning. Always confirm the tray and ring are clean and correctly aligned before diagnosing motor failure.