first off if the carriage still moves up and down i would say something is binding on the mast assy, or possibly a faulty outer cylinder.
I would check the rollers, tracks and chains for something binding up.
Please take appropriate precautions in supporting the mast and staying clear of any pinch points while working on this and by all means get a certified lift tech to assist or evaluate the problem. This is a very dangerous problem that could result in injury or death if not handled properly.