Check your home's circuit breaker for a tripped circuit and reset if the microwave will not turn on at all. Alternately, look in the fuse box for a blown fuse and replace if necessary.
Check the area around the microwave for potential radio wave interference and move interference-causing items away, such as television antennae and cordless phones on a 24 GHz frequency.
Clean the inside of the KitchenAid microwave thoroughly with a warm wet cloth and mild dish detergent. Food and grease buildup can cause the microwave to smoke, arc and not work properly.
Ensure the door can close properly and that the "Demo" mode, if your model has one, is turned off. Both these problems can make it look like the microwave is working but food will not come out warm.
Press "Timer" for five seconds until you hear a single tone and the display shows "Demo Off" If the door closes and the "Demo" mode is off, call your retailer or KitchenAid to schedule service on the magnetron.