Air handler in my car won't move from defrost. What's the problem?

Recently when I turn on the heat or vent or a/c everything is directed to the defroster. It seems like the air handler won't move off of defrost. What might be the problem? Fuse? Vacuum hose? I can change from hot to cold, and the fan speeds work fine, but everything comes out of the defrost vents. The car is a 1999 Ford Taurus SE 3.0 V6.