There are several common causes for this. It could be a broken pin on the back of the heater control knob, it could be a broken bowden cable that goes between the knob and the blend door, it could be a mis-adjustment where the cable connects to the blend door, and/or it could be a stuck blend door.
Near the top of the page, on the right is a button called "Search this Forum." Click on that and search on "heater control" or similar term. You will find many threads on how to diagnose and fix this problem, along with lots of pictures and illustrations.