You may need to take this to a motor trimmer, with out seeing exactly what has failed it is some what difficult to say with any certainty, maybe if you posted a few clear pic's this may help.

Here is a close up of the back of the arm rest. At the back there are two arms that can move back and snap back in place. This is what holds the arm rest in place. It popped off and now I can not put t back on because the two arm can not be pulled apart