Defrost is the default position on the air damper, which directs air to defrost, floor, vent, mix, etc. It's controlled by either electric motors or, more commonly, vacuum lines. It's likely:
A) Vacuum line to the damper has broken or come off.. easy fix, just replace the hose B) The motor or vacuum solenoid has gone out and will have to be replaced. I can't say exactly where these are located on your vehicle, but many times requires pulling the dash which is around $600 in labor.