When you turn the system on do you hear the electric fan spinning? If not, then check for a broken electric circuit to the fan.

This could be a fuse or a connection that has corroded or fallen loose. Check the connectors from the fuse box to the motor housing.

If you do hear the electric fan spinning, then the problem may be that a duct door which deflects the air is stuck in a closed position.

This may be caused by a failed actuator. The actuator may be vacuum activated. Check for loose or leaking vacuum lines from the manifold to the heater-A/C ducting. Also check for loose or disconnected vacuum lines from the unit control head -- the part you use to control the heater-A/C system on the dash area.