The glass probably popped loose from the mouting pads, sometimes the adhesive just comes off. I've seen this a lot in domestic stuff. If this is what happened, and you can see where the pads were bonded on, it can be repaired for pretty cheap. You'll need some glass adhesive, which you should be able to find at an autoglass shop, and you just have to take the door apart and bond/clamp everything back together. Some cars it's damn near impossible to do and some are very easy. Take the interior door panel off and see if you can get a good idea of what's going on.