You probably have a blend door actuator issue. Look just to the right of the gas pedal on the air plenum there is a small actuator down there. You should see the white plastic gear that comes out of it move when you change the settings (defrost, floor, dash, etc.). There also should be 2 spring loaded arms that ride off of it. Make sure they are all moving. The most common issue is a cracked gear, but sometimes the actuators just burn up. If you have to remove it move the tab that is rides on back and forth by hand to make sure it isn't stuck. Take note of the position to make it easier to put the actuator back in.