Like Mark says, could well be a cable break, it is quite common, usually at around the point where the cable goes into the top of the handle.
However, if one of the belts is loose, you could have a loose bearing, often caused by the screws that hold the bearings in place not gripping properly in the body of the chassis. Check all the screws along the line (7 or 9 if I remember correctly). Unfortunately, if the fixing in the chassis is damaged, your "screwed" so to speak.