Open the microwave, and reposition the glass plate on top of the plastic support; the turntable could have been knocked out of position while loading or unloading food. The plastic support fits over the drive-shaft that pokes through the bottom of the microwave.
Heat your food using a smaller plate if the current one scrapes against the sides of the microwave or puts pressure on the back of the door. The turntable might spin underneath the plate, fall off its support or get jammed if you use unsuitable cookware.
Push the door of the microwave until it clicks shut; the magnetron and turntable motor won't start cooking until the door is safely sealed. Something might be holding the door open or you might not have applied enough force the first time.
Some microwave ovens let you temporarily disable the turntable using the options menu; ensure the turntable is switched on for your cooking cycle. The turntable often comes back on after the end of the current cycle.