Tonight my GE turntable microwave has stopped heating. The outside of the machine gets quite warm, but inside everything, including the food, is lukewarm. Is there anything that can be done about this? This appliance is very important to me. Thanks for the help.

Poor heating is usually a sign that the magnetron has failed. I don't recommend replacing it yourself because of the hazardous voltages (about 3000 V) and microwave leakage issues. The part is not cheap ($60-$100).

You should check whether the fan is rotating at the top of the oven.