Microwave turns on but will not heat up - GE Microwave Ovens

While the magnetron might be the problem, further testing should be done before investing in something that might not cure the problem.
I've repaired several microwave ovens that had either a bad hi-voltage diode or hi-voltage capacitor. I always replace both of them at the same time since they can be intermittent and sometimes test good and then later fail, These are much cheaper than replacement magnetrons that might not solve the problem.
Make sure you look for the obvious; loose connections, fuses etc.

The problem is that the magnetron circuit has failed. It is the magnetron that generates the microwaves to heat the food. Typically the magnetron shorts out. The symptoms is that when it is on, it creates a loud buzzing noise when it is shorted. The cost of the replacement magnetron is almost as much as the oven, and not cost effective to repair.