Unless the operating wire has become unhooked, or something is broken, most likely is that the outer sleeve clamp has come loose.
I think you can see the levers without removing the centre console.
Check one of the levers under the dash on the heating unit moves as the hot/cold lever is moved.

To adjust it if the setting is wrong or the clamp is loose, set the dashboard lever 2mm up from 'cold' and push the lever under the dash to the cold position, then tighten the clamp bolt.
if it isn't something in the hot/cold lever, I wouldn't have a clue where to start.