The hinge pin is threaded to aid in installation or extraction. There is no screw that fell out.
That hinge and latch is easily the worst POS ever "designed" by Dahon. Dahon have innovated many excellent things down the years but this is not one of those. A large number of things can and do go wrong on them, all of which result in a wobbly stem riser, and if you ride like that, possibly a broken off stem at the hinge pin.
My best advice is to study the mechanism in as much detail as possible, to understand exactly how and why each piece works, and keep it adjusted in perfect condition. A good place to start is to find a spare hinge kit's instructions.