issues with speedo

Looking for help:
today the speedo of my Trophy 1968 stopped working. I noticed that the needle is stuck, but also the numbers registering the miles are not advancing. I checked the and tightened the nut of the cord at the wheel, but still the instrument is not working. I feel it is not the speedo itself but something else. What do you suggest to check and how? thank you for helping!

Disconnect the cable at the speedometer. Put the bike on the center stand and rotate the rear wheel. If the inner cable rotates
with the wheel the problem is the speedometer or the connection of the inner cable into the speedometer. If the inner cable
does not rotate with the rear wheel, then possibly the speedo gear at the wheel may have died, or the square end of the cable
going into it may be stripped, requiring a new cable.