Long screw that wont come out
Try (this may well be awkward, and will probably need a pair of locking pliers rather than a screwdriver - assuming it unscrewed far enough to get those on before it stopped) pulling down on the screw as you unscrew it. Whatever it's screwed into (a plate, or a nut) is now turning with it - you are trying to get the plate or nut to jam so that you can continue to unscrew it.