heaters are usually controlled by actuators that rely on vacuum lines .. there is probably an actuator to allow heated water to flow under the hood, and an actuator that controls a 'door' in your venting system .. and usually the vacuum lines are connected to the control panel under the dash .. so i would start by seeing if when you turn on the heat, the actuator on the water line under the hood operates and troubleshoot back, unless you know youve been under the dash or something and could have possibly broken a vacuum line ect ..

low coolant can also cause no heat out of the heater.

good luck

tim

So what have you done so far to fine the problem.

check that the motor is operational