Does the air flow direction lever feel very stiff or very light? That lever adjusts a cable/cog arrangement to move the air flaps around.
Its probably either the cable has snapped, come off the fixing point, or a cog has fallen off.
On mine, if you take out the stereo you can see the start of the cables, so check there first. Then whip off the centre console and passenger shelf to see the bottom of the cables and cogs. Haynes details this quite well.