Check vent position actuator at the far left of the HVAC box, which will be above the gas pedal. It has two sliding linkages on it that are spring-loaded. Sometimes they fall off, and sometimes it's because the "peg" on the end of the flap breaks off. Most commonly though they just get gummed up and stuck from old lube on them. Lay down some newspaper so you don't mess up your carpet, spray some WD40 on them and see if you can *gently* work them a little bit; each linkage is two flat pieces of metal with plastic ends and a spring in between. Get them moving and spray a little more. Try your vent position controls and see if they attempt to work. Once you get them moving wipe up the excess WD40 and give them a little shot of fresh white lithium grease.
If it's not sticking linkages, or broken/jammed door, it can be the control head itself - the 1995-up controls seem to be a bit notorious for issues like this.
Richard
