The most common cause of a heater and air conditioner blower not working on a car is a faulty blower motor. Without the blower motor, the system is unable to push air through the vents inside the vehicle.
Another cause of the heater and air conditioner not blowing inside the car is a faulty transistor. If the transistor begins to fail, the system often blows only on the highest setting. A clogged motor or fan can also cause operation issues. As dirt and debris build up on the system, this can cause the components to seize up or work less than optimally.