65 Dart Speedo cable replacement

To cause what you saw, The cable end had to push hard and sideways on the instruments input. The spinning magnetic part contacted and carried the drum/needle part. I've seen them break that little shaft and drop the needle behind the lens. We know this instrument is worn, lets hope it isn't now too far gone to work right again.
Pull the inst' panel and clean the crud from the instruments square drive hole. Route a new cable to it as straight as possible making sure that the cable isn't pushing on the instrument in any direction. Snug the nut which aligns the sheathe/cable.
Good luck with it.