does it work on other settings and is it a push Button or does it have a handle that you pull or lift,if it works on other settings and its a push button then it is 1 of 3 things your salector switch is bad,your vent fins inside the vent ducts are stuck in place witch my becaused by drit or something that fell or crawed in the vent,or if it has a vacume line on the back of the ac/heater controll box there might be 1 thats not hooked up or fell off or broke,if it the kind thats got the handle that you pull or lift then it could be 1 of the last 2 above or the handle might be broke or came off in side or on back of the controll box