While there are many reasons a GE Spacemaker microwave stops heating, the first step in finding the problem is making sure the unit is plugged into an active power outlet. Use a lamp or other electrical device to make sure the power plug is active. Many outlets in the kitchen have GFI plugs that turn off power when they detect a problem and stop the unit from heating.
If the unit is receiving power but still does not heat, a common problem is the door latch is not engaging when the unit activates. Food particles around the door seal often build up to prevent proper sealing and cause leaks. Clean the seal and area where it seats, paying careful attention to the hooks of the latch. Try heating again.
While there are many online sites that offer tips to repair a microwave, GE recommends do-it-yourself consumers not work on these units. Microwave ovens have a high-voltage capacitor that remains charged even if the handyman turns the unit off and unplugs it that creates an electrical shock hazard. These appliances have complex circuitry that is not consumer-friendly. They also require specialized tools that most individuals do not have in their toolbox.