Well you basically just check if all the buttons are working correctly and then check if the wires are pluged in correctly. Sometimes the wire just lets loose for the fan switch and we have a panic attack. But you can also just check if the power switch is on.
Check to see if the blower motor spins easily with the power disconnected. If it does not, replace the motor. If it spins freely, check that the relay that supplies power to the fan is pulling in. Replace that if it is not.