Ac only blows hot air

I have a 2000 ford taurus with climate control. No matter what the setting is, it will only blow out hot air. There are no leaks and the freon is fine. willl not even blow outside air, only hot!. One mechanic said he reprogrammed something but couldn't guarantee how long it would work for. It worked for a couple months blowing cold air but quit working again. Another mechanic said that was not possible that there was nothing to program. When the problem first happened I used to be able to play with the controls..... move the climate controls all the way up then down, on and off, and eventually the AC would kick in..... but not anymore..... just constant hot air. Help it is about to get hot here in houston