My 2003 Silverado Z71 did the same thing to me on the way home from work a couple of weeks ago. I drove straight to the dealer and complained because the truck was only 6 months old.
I was told to drive it back to the service department and they would take a look at it (No cost, under warranty) but the smart ass sarcastic service reps said I would have to make an appointment and leave it there for the day. I was annoyed that I would have to schedule time off from work to have my NEW truck repaired.
When I left the dealers lot, it was working again and I haven't had a problem since.
The next day, I was playing around with the temperature control, moving it back and forth from hot to cold and I could hear the air damper door making a scraping sound as it was switching from hot to cold, so it sounds like my problem may have been a stuck diverter (mixer door, damper or whatever you want to call it)