they have a stud on the bottom that passes through the vent
window fixed frame. There is a compression spring, a nut and some
friction washers. You tighten the nut to adjust the tension on the
spring, and hence the friction that holds the wing from flopping.

Either the nut has come undone, or the weld holding the stud to the
moving half of the frame has broken. You will have to take the door
aprat to find out which it is. If the nut is loose, that should be
evident, and fixable without removing the entire wing window assembly.

If the weld is broken, you will have to remove the glass and re-weld
it.

Good luck,

Gord Richmond