Same thing just happned to me.
I am in the process of makeing a new cat. using one of my old cat. names. Making that one the default. Then deleting the apporiate old cat. that no longer has a name. All the posts then transfer to the default cat.
Glad I only had 30 cats. and not 101!!!
I looked at other messages like this and they recommended doing a restore of your database then upgrading it by increments.
I did not want to mess with that, plus didn’t feel like messing with my host, so I did what this said:
http://blog.cumps.be/wordpress-26-upgrade-fix-missing-categories/