You've been countershafted!
I had exactly the same thing happen to me on an old 88 1VJ. At the time, I was exitting a long tunnel in Italy, with three lanes of mad Italian drivers on all sides - not cool! Luckily my chain did not lock up and I free wheeled to a halt.
This is not caused suddenly, it happens over a long period of time. I think in the pre 3AJ teneres the front sprocket attachment with the two retaining bolts is a weakness, in the 3AJ and later models they replace this with a single large bolt to keep the sprocket on.
To fix this properly, you will need to replace the countershaft (I think it is called) - a fairly big job, as it means taking the bottom end apart.
...ships are safe in harbour, but that\'s not what they were built for...