You have a vent blend door that is not opening either because it is hanging up in the air duct assembly or a bad vacuum hose going to the vent vacuum motor that opens the vent door. I would first check the hoses running into the controls through the firewall in side the engine compartment for leaks or cracks. If all is OK there, you'll need to remove the control panel and check the hose connections in there. Finally, if all is OK with the hoses, you'll need to get into the ducts and check the blend doors.
All cars are different and especially with respect to the air duct system. The blender doors are like little flaps inside the air duct system that open or close to redirect air to the footwells, vents or dash. They should be pretty easy to identify. The hoses in an HVAC system run first from the engine intake manifold through the firewall and then into the control panel. From the control panel there are vacuum hoses to the blender door actuator motors. To be honest, you really need to buy a Haynes or Chiltons repair manual to do this type of work as it will have pictures and step by step instructions. They cost about $25 at any auto parts store.