Air not blowing out on windshield, will not defrost.

[little_lulu3]

Hi everyone! I know nothing about cars but my Dad is helping me out a bit to figure out what is wrong. Now that winter has come I have noticed that my windshield air vent isn't blowing any air out when I switch the knob to windshield. In fact when I switch the knob to anything (floor, windshield, face) it seems to stay on the two vents on the sides and on the floor. My Dad thinks that it might be the actually knob that is broken and is maybe it is stuck on face and floor. It seems like it might be loose compared to the other 2 knobs but I can't really tell. Has anyone had similar problems? Is it the actual knob or do I have a vent problem? It's become really dangerous now because now the windows fog up and freeze and I can't see anything. Thanks for any future help!
Oh also, I have a 2006 4 door hatchback.