Take the middle part of your dash apart. Pull out the entire ac switch assembly (do it with gentle hands involving a lot of prying because otherwise small plastic hooks on the vertical side covers can break off. A quick tip: start peeling those vertical covers off from the bottom side). You can carefully open the push button circuit and pull it out. It is held together with small plastic hooks to the main switch assembly. You will need a small flat head screw driver to pop those hooks open and the entire knob will come off. There will be a small piece (1 inch by 1 inch) of circuit board inside with a button on top. It is connected to the rest of the circuit by a very small plug-in type connector cable. In my car, once this happened and it turned out that the cable had come off. So I did the same that I told you here. I simply lubed all my switches and plugged the small white connector cable back into the circuit board securely and that fixed the problem for good.
One more indicator: If you turn on the lights in your car and you are not seeing the red glow on your ac switches, consider this problem as confirmed because unless all 3 circuit boards for all switches are not properly connected to the main circuit with their respective ribbon cables, the red glow lights don't light up.