Could be numerous things.
Thermostat stuck open, faulty water valve, heater core clogged, temp control cable broken/disconnected,blend air door.
Is the coolant level full? Is the engine reaching operating temp? Are both heater hoses hot before and after the water valve?
If both hoses are cool you have a thermostat problem, if 1 hose is hot and the other is cool you have a heater core problem.
If both hoses are hot the problem is in the car, remove the control head, be sure the cable is attached on both ends and that the blend door actually moves.