need help my whole unit is stuck on defrost ?? what could be the problem?

All the valves under the dash are vacuum controlled, and the default when you have no vacuum is to defrost. You probably have a vacuum leak. The vacuum line that goes to the AC controls is a small line, about 1/4" diameter that runs down the firewall from the vacuum tree on driver's side inner fender to the passenger side of the vehicle. Usually that's the line that gets a leak in it.

Kurt