If you lift up on the seatback release, does the handle move freely without any resistance? If so, then I bet the small little wire cable attached to the seatback release handle that disengages the seatback lock snapped.
If you can get the cover off of the back of the seat (a couple of screws on the bottom of the seatback hold it on), you can look in there and see where the cable should run and if the cable has snapped.