This has got to be the most frequent problem that comes up on this website. All the control valves are vacuum operated, and defaults to the defrost vent when the vacuum fails. There is a vacuum tree on the inside fender on the driver's side. There is a small black vacuum line that runs from that vacuum tree across the firewall to the passenger side, and then through the firewall. 99% of the time, the vacuum to the HVAC fails because the heat of the engine bay caused that line to fall apart. Check for a leak or breakage on that line.
Kurt