If you don't know about the dangers of microwave ovens, do a little bit of research before opening it up. They can be among the most dangerous appliances to work on due to the high voltage power supply, which is capable of producing a substantial fraction of an amp at a few thousand volts. It's a very definite electrocution hazard if proper precautions and care are not exercised, and (particularly if the circuitry is malfunctioning) can carry a dangerous charge even when not powered up.
Having said that, it's certainly possible to be safe and sometimes to repair microwave ovens. Whether it's economical these days is a rather different matter, but that depends on what the problem is. A nonworking fan might be something as simple as a mechanical binding or buildup of crud.