Unstrung means to remove a string. A violin is an instrument that is comprised of strings. To be made "unstrung" means to be made weak or disordered. The joke is playing on the word "unstrung" and its different meanings. It's clever that the word "unstrung" was used on a violinist since that is something you can do to a violin.
OLD VIOLINISTS never die , they just become unstrung .