Every WP blog suddenly lost all categories!

Ack!
Please help! (If you can)

I tried to upgrade a 2.86 wp blog.
It didn’t work and it lost all categories.

Now, for some reason – EVERY other WP blog I have has lost all its categories!

All posts, comments etc are all still there.

How can this have happened? I didn’t touch the other blogs at all.
How could the failed upgrade of one WP installation affect the databases of the other blogs?

One of these other blogs is running 2.86 and the other two are running 3.01