Diagnosis
The turntable motor gearbox has failed. Gears inside strip or bind. Food spills can also jam the roller ring.
How to Fix It
First, remove the glass tray and roller ring. Clean the cavity floor and the ring guide. Ensure the ring rotates freely. Reinstall everything properly.
If That Didn't Work
The turntable motor itself is faulty. You must replace the motor (often called the tray motor or drive motor). It's located under the cavity floor.
Heads Up
Running it with a grinding motor can overload other components. Stop using it until fixed to prevent further damage.