Diagnosis
The little motor that drives the turntable has either failed, or the coupling between the motor and the roller ring under the glass plate has come disconnected. Sometimes it's just a piece of food debris jamming the mechanism.
How to Fix It
1. Take out the glass plate and the plastic roller ring underneath. 2. Look down into the cavity floor. You'll see a small, white plastic drive hub in the center. 3. Manually rotate that hub with your fingers. If it spins freely with no resistance, the motor is likely dead. 4. If it's stiff or won't turn, there might be a piece of burnt food or a stray popcorn kernel jammed in there. Clear it out.
If That Didn't Work
If the hub turns but the motor doesn't spin when you run the microwave, the turntable motor itself has failed. It's a relatively small part that mounts underneath the cavity floor. Replacing it requires removing the microwave from the wall and disassembling the bottom.
Heads Up
You can technically use the microwave without the turntable spinning, but your food will heat very unevenly—hot spots in the center, cold edges. It's best to get it fixed.