Sounds like the airflow direction control knob has come unhooked from its linkage. Your best bet is probably to open up the dash and to have a look at the proper linkage, at the back of the HVAC module (the one with the three knobs). It'll be one of the three cables - either green, yellow, or clear - can't remember.
Open up the dash, starting with the top trim piece in the centre of the dash. It just pulls right out. Then, work your way down to the HVAC module, removing trim pieces as you go until you get to the HVAC module. (You might or night not need to unscrew the radio head unit from its housing.) The cable is usually held in place on the back of the HVAC module with a pair of flimsy plastic clips. These can break easily. I glued mine in place. Just make sure the glue doesn't interfere with the movement of the linkage inside the plastic sleeve.