It seems likely that at some stage it has been pushed up enevenly - this can cause one "loop" of the cable (on one side, right or left) that is wound onto the cones on the ends of the spring above the doorway to come off. To solve this the best way is to push it up unevenly again (lifting the droopy side) and winding an additional loop onto the cone. Close the door completely (perhaps needing a up-and-down rigmarole to achieve this, mind the cable is free to unwind) and then reopen, with luck everything will be back to normal. Beware of one thing though: the cable may have come off the cone (now or earlier) and been damaged. To check this close the door completely and inspect both cables - spare cone with cable used to be available (there is a bit of a trick to replacing these - consult instructions and follow carefully). In the worst case, the cable may have snapped, but I think you would not then be able to open the door any more.