Eliminate the cable 1st as already stated, but it could also have just come loose from either the trans (not likely) or the speedo itself. I have seen the cable come loose from the back of the speedometer a couple of times.
If the cable is still connected and spinning it slowly with a drill does not move the speedo, the cable is the most likely issue. But if changing the cable does not help it is the speedo itself (only seen that 1 time) after that it is either the trans drive gear or driven gear, or both. The driven gear is usually easy to change (but not always, depending on the type of trans you have) the drive gear requires at least the trans tail housing to be removed and may or may not be somewhat easy to change, again subject to the trans type you have. It also get somewhat complicated to determine what ratio drive or driven gears you need.
Lets hope it is either a loose or Bad cable as that is the cheapest & easiest fix.
Please let us know what you find.
