I have zero FHE on Ducati mechanical cable drive speedos, but FWIW:
Perhaps the ringnut has come loose on the bottom of the speedo, and the cable wire has disengaged from the speedo input hub.
If that was OK, I'd unscrew the ringnut and pull the cable away from the speedo, and turn the front wheel to see if the cable wire is turning.
If it's turning, your speedo may have given up.
If it's not turning, it may be disengaged at the wheel, broken, or the drive gear mechanism in the wheel may be broken.