Could be a busted cable, could be a broken cable bracket. Probably won't be able to tell until you can get the headliner off, and unfortunately this requires being able to partially open the roof.

I half fixed my sunroof about two years ago - one lifting angle was completely busted and letting the roof flap in the draft on the highway. I fixed it to the point where it would stay shut (and keep water out), but never quite fixed it where it worked they way it's supposed to. I may get around to fiddling with it again this summer.