DIAGNOSIS The turntable motor has failed or the drive coupling has stripped. This is a common wear item.
FIX First, ensure the roller guide and glass tray are correctly seated. If they are, unplug the unit. Remove the glass tray and roller guide. Look underneath the cavity floor at the plastic drive coupling. If it's cracked or the teeth are stripped, replace it. If the coupling is fine, the turntable motor underneath has failed and needs replacement.
IF THAT DIDN'T WORK A wire may have come loose from the turntable motor terminals. Check the connection.
HEADS UP Do not operate the microwave without the turntable and roller guide installed. It can cause uneven cooking and damage the waveguide cover.