sounds like the little plastic "winged" bolt thingy holding the pane on to the runner has snapped,
will need to take of the door card (easy, just undo all screws around door handle, and lower pocket, pop off door lever srround, and ease away from door) then there should be a membrane over the metal frame, peel that off, and the window mechanism should be exposed.
be careful here, the metal door frame is quite sharp, and cuts are easy to get
wind down the window so you can see the horzontal bit the glass pane rests on, in the middle should be a white plastic plug, on the other side should feel the 2 wings popping out, holding it in place. the pane should be a tight fit into the runner it rests on, and can be hard to move.
if the plug is broken, pop it out, replace with another one (should be pennies from a dealer/scrappy) and replace all the door trim
voila! btw this worked on my tub with electric window, feel free to modify for non tub windups