My microwave is no longer putting out any heat

Not many people will repair microwaves because they are mostly an uneconomical repair when the magnatron or diode goes.it is best,cheaper and safer to replace.

The problem could be caused:
top door micro-switch
or
Answer: First, make sure your microwave is plugged in and getting an ample supply of power. If the power is on, but it does not work check to make sure the door closes and aligns properly. Microwave door switches have metal terminals on the end that make contact with the door hook or latch. This completes the circuit which allows the microwave oven to run and without this contact the microwave will not operate. A simple misalignment might be breaking the circuit causing the microwave not to operate.
If you suspect that the thermal fuse has blown, call a professional appliance repair person. Even an unplugged microwave oven can kill or seriously injure someone. This is because there is a high energy capacitor inside the microwave oven case. If it is not discharged properly, you could be electrocuted.
If your microwave buzzes or hums but does not produce any heat, it could be a faulty defective diode, triode, high-voltage capacitor, or even a faulty magnetron. However, this problem usually means a bad means a bad magnetron, followed by a bad diode. Call a professional appliance repair person for assistance.
Warning: Microwave ovens are probably the most dangerous of consumer appliances to service due to the very high voltages (up to 5000 V) at potentially very high currents. If you have any doubts about your knowledge and abilities, replace the oven or have it professionally repaired.