Air conditioning blowing through the wrong vents

Air conditioning working fine one day and the next day it will only come out of the defrost and floor vents. It does not matter where you direct it via the buttons. It is a digital setup on the dash. Even if you attempt to make it only blow out of the defrost vents it nonetheless chooses to go out the floor and the defrost, likewise if you choose to make it go out the floor vents, it chooses both top and bottom, but never, no matter how you choose to direct it, does it ever come out the main dash vents. which is where you want it in this 109 degree weather.
What is causing it and where do I look to fix it?

Thanks a ton, you guys are lifesavers, never failed me yet.