Damn it's hot in a car with no a/c.

I had fully working a/c until a few days ago... I turned the heat on and herd a pop like I broke something behind the knob... I've looked and looked and can't find anything wrong, but when I turn it back to cool I get hot air still. The compressor and fan are both coming on as well and seem normal.
Any Ideas?

Thanks for your help guys (and gals)!

 

Does the knob turn without resistence now?
check the heater control vavle on the firewall inside the engine bay. Maybe the cable poped loose, or check under neath the dash and check the cables. You might even have to pull the control head out to inspect.

 

It has resistance, but doesn't seem as much. As far as I can tell, the knob only controls one cable that is working properly. The cable moves an arm on the bottom of the evaporator box. Is there a second cable or electric switch I should be looking for?

I'll look for the control valve and see if it's moving properly in the morning. I'm trying not to take the controls out because I would have to disassemble a big portion of the dashboard ... which will suck.