I just had my axle housing R&R, used rear end replaced and my axle came apart what would cause this?

How does an 'axle' come apart? .. The axle itself is solid steel..!!!.. Do you mean the axle broke? .. Did it come out of the rear differential housing? .. Did the axle bearing fall apart? .. Was any lubrication added to the replacement differential upon installation ?? .. Any warranty on this repair??? .. The more information we get will help us give a more accurate answer! .. Tell us the story, eg.. I was driving along and.....