Have you pulled apart the dash to make sure the door the controls air flow is not stuck?

 

Your vent doors are vacuum operated. The odds are very good that yo have lost the vacuum to the system. Do you have a Haynes or chilton manual? Better yet, do you have a Factory Service Manual? I would suggest getting them if you don't. The vacuum lead for your system comes through the firewall low on the passenger side. Locate it under th ehood first and then have someone move it around a bit for you so you can see it more readily under the dash. Make sure it is intact and connected on both ends. Also, check out your entire vacuum system, connections and accumulator bottle. start your engine and spray each of the connections with an aerosol carburator cleaner and wait about 10 seconds. If your engine RPM surges slightly, you have a leak at that point - fix it. If it doesn't surge, go on to the next one. There are a dozen or more to check.