There is a small chunk of green plastic that holds a small cylinder shaped piece of metal on the wire that runs your window up and down. What happened to my car is the cylinder broke through that plastic and when you roll down it still pulls but because the cylinder is above the plastic has nothing to grab when you try to roll it up.
I did a lot of search before being a member here but this is a good DIY that I used to fix my rear window so I did not have to replace the entire regulator. It even has some pictures to help you through. I did not cut my door though. There is plenty of room to do this w/out taking snips to your metal. Takes about an hour, dont be afraid to pull your panel... give it some effort and it will come off.
BTW, make sure you glue the piss out of it w/epoxy and let it dry ALL night long. If you don't, you will end up replacing it the next time your friends SLAM your door! ugh people never listen when you say gently close!