heater doesn't work

Does the engine reach operating temperature? Zee

As Zee asks, if you drive the car 5 miles or ten minutes or so does the temperature gauge read toward the middle or does it stay down toward the bottom. If so the cooling system thermostat (like the thermostat in your house) may have failed in the open position not allowing the heater work efficiently. Check the coolant level if it is low perhaps you have air in the cooling system.