My air or my heat won't work. It went out suddenly and without warning. What? on

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My air or my heat won't work. It went out suddenly and without warning. What? on 2006 Ford Crown Victoria I've had the car going on 3 years. I bought it used. The air never blew through the vents, but it would blow throught the floor or defroster. The heat worked find. But now my air or heat won't work. What is it, and what can I do and how much do you estimate the repairs to be, and where in my air is advised?