If there is cold air blowing from some vents and not others then one of a few things:
1) If the cold air blows from the defroster, you move the selector to the regular vents and it blows hot air, your a/c freon is low.
2) If no air blows out of the dash vents after you move the selector, there is a vaccum leak which doesn't allow the door to re-direct the air or there is a problem with the internal door itself
Good luck, shouldn't be too hard to track down the problem