WD40 is made from fish oil. It evaporates with time. The propellant is a volatile solvent. Check manufactures specifications. Different oils and solvents may dissolve coatings and components on circuit boards.
There should be no water from outside on the circuit board. If so, clean it up and seal the leaks.
As suggested above, corrosion and vibration are the usual suspects. Since the fan won't spin that narrows it down a little. Maybe Old Biscuit will give detailed trouble shooting instructions. Possibly it is the thermostat or thermostat wire connections.
I wish you good luck and happy trails ahead!