How can I tell what needs replacing on window which falls down?

You'll need to get inside the door and find out what broke. Or have a reputable shop do it for you and assess the damage.

that happened to me..the track separated from window..sikaflex 220+ direct glazing adhesive..worked for me did it myself,saved myself probly coulpe hundred bucks..on the bottom of window ther's a metal piece that works with the sissor like arms that lift ur window..my came off the bottom of the window..hope this helps