Hioridge - My speedo suddenly quite. I unscrewed the cable sheath from the bottom of the transmission, pulled on the cable inside it, and it came out. The cable twisted into two pieces. This would be a quick check.
If that's not it; Tom is correct the cable sheath needs to be removed at the speedo to remove the cable.
My speedo became increasingly hard to turn and twisted the cable. I repaired the speedo by using instructions I found. Removed all the old grease, and lubricated all bearing and gear surfaces. Caution - the speedo must turn VERY freely to operate properly and not continue to twist the cable. (I had to buy another cable because I had not freed it up enough.) It takes a lot of cleaning and lubing (and time) to get the speedo to spin freely.
There are numerous speedo repairers, and they are very good at what they do.
Good luck,
Ron/IA