the defrost vents are on the top of the dash, an inch or so behind the windshield. The top register, above the register with the wheel control, corresponds to the top lever on the climate control; the middle one controls the wheel one and the ones that go through the doors, and the bottom lever controls the footwells and the rear.
Make sure all the climate control levers are over to the right, and the wheel is all the way up into the red. Turn it on and check for heat. Now close the top and bottom lever, and close the vents on the doors using the thumbwheel.
If any air is coming out now, it should be hot. If not, perhaps the bowden cable that cotrols the flap has detached, or perhaps one of the stepper motors has gone out.