This is probably a problem with your dashboard switch unit whch defaults to the windshield when there is a problem. New is expensive but many are available used.
Inside the rug covering the transmission tunnel, on the passenger side below the glove box, is a bank of vacuum actuators that open and close the flaps that direct the air flow. Each actuator rceives an electrical signal from the push button unit telling it to open or close vacuum. The flap won't open if it isn't getting the signal. It is possible that the actuator, itself, is getting the signal but isn't working or that the interor vacuum element is not working. This can be tested by disconnecting the vacuum hoses from the actuator and applying vacuum with a hand pump or by connecting the vacuum hoses together in which case the flap with open and you will get heat to the floor.