If the microwave doesn't run at all, the power cord, fuses and door switch should be checked and replaced, if needed. If the unit needs to be disassembled for repair, it may be best to hire an expert as microwave capacitors can store a significant amount of electricity.
The first step to diagnosing a broken microwave is to inspect the power cord. Over time, heat can melt the plastic casing of the power cord and damage its wires. It's also worth using a tool to check the outlet to make sure it's producing sufficient electricity. Uneven cooking may also be caused by a faulty wall outlet.
Microwaves only run if the door is closed, and they use a switch to make sure the microwave stops running if the door is opened. The position of these switches varies, but it should depress with little force and spring back once released. A replacement switch can get the microwave running again.
Some microwaves also have fuses accessible without opening the unit, but many require removing the outer case of the microwave. If removing the outer case reveals capacitors and wires, however, expert help is recommended to avoid potentially dangerous shock. Replacing a blown fuse is likely to get the microwave running again, but electrical problems can cause more blown fuses in the future.