More than likely the air vent door for the heat is not working. There is a small motor called an actuator that operates the door and it may need replacement. Sometimes the doors will get stuck as well. The other possibility is that there is air in the cooling system. To bleed air just remove the cap when engine is cool. Turn the heater setting to all the way hot. Run the engine until it reaches operating temperature and top off radiator and reserviour.
If it is the air vent door you will have to locate the actuator that is not working by going into the air box, under dash center, and finding which air door allows hot air from the heater core into the system.
I am giving you a walk through diagnostic to also help with this problem. Just follow the link below;