[Bassasasin]

I believe, as most do, the arms connect to a spline shaft.
The wiper must have slipped on that spline wiper driver shaft.
When I take off a window or something I push out the release at the bottom of the arm and release it off the spline.
Sometimes they are pretty stuck from dis-similar metal galvonic response.
However, the arm's socket is usually a softer pot metal and the whole wiper arm proubably needs replacing from a junker.

Good Luck