If the vent control knob still adjusts the flow of air everywhere else, then the pipe to the window vents has come off.
Get down under the dashboard and look up, and using a torch look where the vents are on the dashboard, you will see the entry point to the vent, which the air pipe connects to, and the disconnected pipe.
Just push it back on to the vent. If it's still loose, place a cable-tie around the hose and tighten.
Davymac, December 2009
