The magnetron is the component responsible for creating the microwaves. If this unit is not working properly, then your GE Spacemaker microwave won't heat properly. Fill a bowl up with 1 cup of cold water, and place it in the microwave. Run the microwave for two minutes at the highest heat setting. If the water isn't warm, then the magnetron is defective. If you don't have the proper training, do not try to replace interior microwave components. Your microwave can shock you even when it's unplugged. Contact a qualified microwave technician.
If your home has two fuses instead of a circuit breaker, see if either of the fuses is burned out. Sometimes your microwave will run even if only one fuse is working. It may not, however, heat properly or at all. Change out any blown fuses. Plug the microwave into its own circuit. If you do not, another appliance may draw too much power away from the microwave, causing it to work improperly.
Additional Heat Issues
If your microwave sounds normal, but doesn't heat, you may have an issue with the circuit board or circuitry. If the microwave doesn't heat, but makes a lot of noise, you have an issue with either the high-voltage capacitor or power diode. These are difficult and sometimes dangerous repairs for consumers. Contact a qualified technician to assist you with this issue.
If your microwave is not powering up and heating, verify that the power cord fits completely into the outlet. Reset the circuit breaker, if tripped, and change out blown fuses. See if the door is completely closed, and remove any items preventing the door from closing properly. Clean the door gaskets if they have debris on them, or get new gaskets if the currently installed ones are torn or extremely worn. Examine the door latch and release button. If either of these components is broken, replace the door, since these parts are difficult to repair. Check the internal/thermal microwave fuse. Unplug the microwave from the power supply, and remove the main housing. See if the fuse is blown out. If so, contact a technician to assist you with this repair.