Did you turn the direction dial to vent? I assume you did if the other vents are working ok.
Sounds like the pipe may have come off the heater blower behind the dash for some reason?
Remove the lower part of the dashboard under the steering wheel (6 screws from memory) and have a quick look (you're looking for a black plastic pipe that runs from the vent to the heater blower).
Also put the blower on and have a feel about with your hand while you're there and see if you can feel air escaping from anywhere, may be a hole in the pipe or some kind of blockage.
EDIT: And welcome to the forum!