Microwave turntables rotate the food in the microwave while cooking it. Microwaves are radio waves created by a magnetron. The food is cooked as it absorbs the radio waves, converting them into heat. The turntable ensures that the food is cooked evenly as the microwaves only come from one part of the oven.
Check that the glass tray doesn't wobble. If it wobbles, that means it isn't positioned correctly on the roller ring. It can also mean there is food under the roller ring which is making the turntable turn unevenly. Take out the glass turntable and the roller ring. Wipe them with a damp cloth.
Check for noise coming from the glass tray. If the oven is operating and you hear noise it could be because the roller ring and oven bottom are dirty. Clean the oven bottom and the ring. Use mild detergent water for the oven bottom, or you can use window cleaner. Dry the surface. You can clean the ring in mild soapy water or dishwasher liquid.
Clean the glass tray. You can use warm soapy water, or just stick it in the dishwasher.
Check that the turntable turns. If it doesn't, try reseating the glass tray on the central turning sprocket.
Remove the ring and glass tray. Place a cup of water on the microwave bottom. Press the beverage setting. Peering through the glass door, turn the oven on. Identify if the sprocket is rotating. If it isn't, call for service.