This happened to me last week.
I simply turned the big dial on the right that chooses where the air comes out a few times and then the hot air started to come out the dash vents. I think, in my case, it was a valve or damper not opening up. It was the first time I used the heat this Fall so perhaps that had something to do with it. In any case it has worked fine ever since that one and its 29° out this AM so hopefully it was just an isolated occurance.