As they say, I feel your pain. Seriously, I had the very same thing happen to our 2003 C2 two years ago. Driving along with the HV/AC on "auto" when all-of-a-sudden, nothing but heat!
OK, here is the long story short. Look and see if the arm on the downside of your AC unit under the dash in the foot well of the passenger side is still connected to the flapper valve that controls the split of hot or cold air into the cabin. In my case, the control arm suddenly disconnected from the flapper valve (mixer), so only hot air was coming into the cabin.
If this is the case, be aware that the the entire AC portion that this is a part of is considered by Porsche to be a single unit and individual parts are not available. The Porsche list price to replace the AC unit is astronomical and the used parts are almost "unobtanium" in nature. I tried to use epoxy to re-connect the arm to the bottom of the mixer valve, but it soon failed. In the photo I have attached, you can see the culprit; The arm from the pink part is supposed to connect up with the bottom of the flapper valve. But, as you can see, there is no connect. The end of the connecting arm has dropped off of the end of the flapper.
Observe the end of the connecting arm not connected to the base of the flapper (mixer) valve.
Base of the mixer valve is supposed to mate with the connecting arm from the temperature control motor that controls cabin temperature by mixing cold and hot air. Failure led to only hot air entering cabin.
Here is what I did to repair my car: Since epoxy cement failed to hold the connection arm to the bottom of the mixture valve, I went to a hardware store and bought a $3 spring. I stretched it over the connecting arm in such a way that it held the end of the arm into the orifice at the end of the mixer flap. Problem solved for $3! Two years now and no problem with the HV/AC.