climate controls not working

After my car sat for about an hour after running fine, none of the climate controls would work in my car. All I could get was a hissing noise and then it would stop. I have a 94 3.1L V6 cavalier. Any ideas what it is or how much it would cost?

vaccum leak possibly underhood at main harness or even under passenger side dash area. or at the rear of the a/c control head .either way it sounds like a loss of vaccum ..