DIAGNOSIS The turntable motor (drive coupling) has failed or the roller guide is jammed.
FIX Unplug the microwave. Remove the glass tray and support ring. Look in the cavity floor hole. The plastic drive coupling should be square and turn freely by hand. If it's broken or doesn't turn, replace it. Part number WB27X10930 for the coupling assembly.
IF THAT DIDN'T WORK The turntable motor itself (located under the cavity) could be dead. You'll need to remove the outer case to access and test it.
HEADS UP Running the microwave without the glass tray and support ring in place can damage the floor and cause uneven cooking. Always ensure they are correctly installed.