Diagnosis
The turntable motor (often called the drive motor) has failed or its plastic coupling to the roller ring is broken. It's a common wear part.
How to Fix It
Unplug the unit. Lift out the glass tray and roller ring. Look underneath at the small, silver-colored motor in the center of the floor. Check if the plastic coupler on its shaft is stripped or if the motor shaft itself turns when you start a cook cycle. If the motor is dead, replace part number WB27X10030 or equivalent.
If That Didn't Work
If the motor runs but the tray doesn't move, the roller ring or the guide on the glass tray itself is broken. Replace those parts.
Heads Up
Cooking without the turntable functioning will result in very uneven heating. The motor is a low-voltage DC component. The main board sends power to it, but board failure here is rare.