2007 GM a/c heater control not working
 

I have used the a/c within the last week or so. Went to work Friday morning and had heater on low. Was heading home from work Friday afternoon and had turned the a/c on and got nothing but hot air. Switched to vent and got the same. Even on max air I am only getting hot air. Where do I need to start looking for the problem. When I switch from heat to ac I don't here the door (valve) working. Just hate heading into something blind. The car is a 2007.