No air coming out of dash vents

there is a valve that open and closes to direct which source the air comes from it is either stuck or the vacuum that switches it is disconnected somewhere the valve will be under the dash

thanks..I will check it out. Had dash plate out few weeks ago. May have accidently disconnected the vacumn

it may also be electric but is so if when you check it just switch from heat to a/c you should hear it switch the valve usually do the switch it just controls a flap in the venting system http://youtu.be/embUVHD3O4o