No heat from vents

My daughter has a 2000 Concord that has a problem with emitting hot air. The ac works just fine as does the vent, defrost (but no hot air from the defrost) and recirculating of air but no hot air from the heater. I have checked the fuse and looked for hose that may be loose but have not found any problem. The temp gauge reads normal, half way between cold and hot.
Could someone tell me where to start looking for her problem?
Thanks in advance.

could be a blend door malfunction not allowing the hot air from the heater core to be directed properly.

could be a blend door malfunction not allowing the hot air from the heater core to be directed properly.
Would this have to be reached from inside the dash? If so is this a do-it-yourself job? What would I have to remove to get to this blend door?