The window is held on by two plastic lock-nut type things (technical term eh?). Same thing happened to my passenger window, it came off track and I popped it back. I just never opened the window again, but it would start rattling on the highway.
Anyway, one of those plastic retainers probably popped off. It should still be inside your door. Just take the door panel off and look for it and secure it back on. It's pretty hard to do as you have to reach through the limited cutouts in the door and basically do it blind. Adjust the window up/down until you have the best access. Thin socket and pliers should be enough IIRC. Might be a good idea to replace that actual plastic piece, because my window is still a bit creaky but it's at least held in place now.
