Micro wave not heating up

The short answer here is that yes, a microwave oven can be repaired in most cases.

As Theo already stated, we can't do anything without knowing which microwave you have, as repair and troubleshooting procedures will vary widely among different models and manufacturers.

Failing to heat can be caused by any number of problems, so diagnostic steps would need to be taken to determine what failed. Are you comfortable using a multimeter? Can you safely discharge the high voltage capacitor inside the oven? Are you knowledgeable about safety procedures for high voltage circuits?

If you answered no to any of these questions, you'll be better off taking it to a repair shop than trying to work on it yourself. There are potentially lethal voltages present inside so precautions must be taken in order to safely work on it.