As mentioned above, if it has been ridden while loose, chances are the square broach in the arm has been damaged. You can try fixing it, as [MENTION=410984]Trevtassie[/MENTION] suggests above, but in my experience this is seldom successful. A replacement non-drive side arm shouldn't be too costly, though. Unless you're a stickler for looks, it doesn't have to exactly match the old arm. Just try to get the same length as the original.