When you move the cartridge carriage does it move smoothly without a lot of "play" up / down / side to side / back to front ?|
I am thinking that one of the metal bushes either side of the carriage has come adrift. If it has and is the right hand side one, it will be at the end of the carriage bar in the parking area, if the other one then look to the left end of the bar.
It can be refixed by wiping clean and applying a single drop of superglue before wobbling the carriage around so the bush can seated fully home. Be very careful to protect all surrounding parts from the glue.