DIAGNOSIS: A non-spinning turntable. The coupler (the plastic piece under the glass plate that connects to the motor) is stripped or broken, or the drive motor has failed.
FIX: Remove the glass tray and the plastic support ring. Inspect the small, white plastic coupler underneath. The teeth on this coupler commonly wear down or crack. This is the #1 fix—replace the coupler. It's a cheap, model-specific part.
IF THAT DIDN'T WORK: The turntable motor has failed. You can test this by removing the coupler and seeing if the motor shaft spins when the microwave runs. If it doesn't, the motor needs replacement, which requires removing the outer wrapper.
HEADS UP: Running the microwave without the turntable spinning can cause uneven heating and hot spots in your food. It won't damage the microwave, but it defeats the purpose.