For some reason, one or more of the various doors that directs the blown air through the the heater to the correct vents has quit working. The doors are operated by motors and control rods from these motors.
The only way to diagnose and correct the problem is to get access to a factory service manual and follow the procedures for the symptoms you have. The manual has excellent diagrams and flow charts for sourcing the problem. Unfortunately for the most part you'll be laying on the floor of the car to access what you need to get at, and its much easier with the passenger seat out of the car. You may also have to remove the dash from the car to get at some areas. Not a fun procedure. Good Luck!