Its hard to say what it could be, but its likely something with the track. Either the roller has come off the track and something plastic has broke and caused it, or the cable is off the guide...again something broke. Think the only way to find out is remove the door panel and look...anything else is going to be a guess, and you dont want to be buying parts that have to be ordered without knowing for sure. The door panel is not that bad to remove...pop it off and see what you need.
