Line Fuse
Every microwave oven has a line fuse. This fuse is designed to blow if too much current is drawn through the microwave circuitry. The fuse can be replaced, but the underlying issue should still be investigated and resolved.
Main Control Board
If the microwave is not working the main control board may be defective. This is not common. When a microwave stops working the problem is usually the line fuse or within the high voltage circuit; either the high voltage capacitor, diode, transformer or magnetron.
Thermal Fuse
If the microwave is not working the thermal fuse may have blown. It can be tested for continuity.
Here's a link to the parts you may need: http://www.repairclinic.com/RepairHelp/How-To-Fix-A-Microwave/66-3-600264-/GE-Microwave-not-working-JVM1490BD003