Air flow door wont allow air to blow defrost

AC and heat controls work fine. Fan speeds work fine. But air flows only through front vents. When I attempt to change air flow to floor or defrost, the air continues to flow through the front vents. I can still feel air coming from the defrost vents but it seems that it is blocked by the door or flap that will change direction of air flow. I believe a piece broke off when changing from vent to defrost because I actually heard the door fall back down when doing so. I am able to turn the dial to other settings and they work fine. Is there an "easy" way to fix the door and/or the piece that would move it? I hate to put alot of money into it being that it will still defrost after some time but being that its winter I would like to have it fixed. Thank you so much for your advice, much appreciated.