You can start by checking the fan for debris and making sure that your fan s motor is properly lubricated.
How to repair attic fan.
After you realize that it s not an electrical or thermostat problem the chances of it being a problem with the motor are quite high.
Your belt might have deteriorated to the point where it is slipping and not turning the blade effectively.
Have you had an roofing siding or insulation work done before you noticed the difference in performance.
If the motor isn t working you will need an attic fan motor replacement or attic replacement fan.
This attic fan motor is the most expensive repair for attic exhaust fans.
It s best to hire an electrician to deal with electrical problems.
Most fans have an on off switch that bypasses the fan s thermostat.
If the fan comes on you have an electrical problem in the circuit the attic fan is on.
If however the fan still doesn t turn on the problem is most likely with the fan motor or the thermostat.