First of all, make sure there is no air at all in the cooling system. The most common cause for cold heat, is low coolant, or air trapped in the cooling system.
If that isn't the problem, check that you don't have a water valve on the heater hose. Some vehicles do, some don't. If you have one, make sure it opens when you move the slider/knob from hot to cold and back.
If you have enough coolant, and you have water flow, move that temp control back and forth, and check if you can hear the temp blend door open and close. Often times the linkage will break or become dissconnected. There is a small door, usually directly above or in front of the heater core, that should be opening and closing when you move that control.
If none of the controls to adjust airflow are working, check to see if you have a manifold vacuum supply to you HVAC controler on the dash. Sometimes the vacuum line that comes into the cabin, is dissconnected at the engine.
