Yes, sounds like the flap isn't working, I assume its a cable operated system unless it has aircon, check either end of the cable for disconnection.

Does it have electronic controls? If so the flap will probably be driven by a motor, these can go faulty.

It will be on the side of the heater unit, you may be able to get to from under the passenger side footwell, that's where the old Citroen car ones were.