If it is the round vent with a pull down handle, it is a Ventline Vanair fan.
In order to diagnose the problem first disassemble the interior trim. Once the trim is off you can see the wiring.
Use a meter to check to see if there is power coming to the fan.
If there is power coming to the fan, the next step is to check the switch to see if power is going through the switch. Rotate the knob then check the wire going to the motor.
If power is going through the switch, the only thing left is the motor.
If you are paying someone to diagnose the problem then replace the part, it might be better to replace the whole thing. A new fan is not expensive.
This fan (link below), a tube of caulk, and some butyl tape is all that is needed.
There are some YouTube videos that show how to do the install.
Here's one for an Airstream> https://youtu.be/drG8stwFbLc