My categories and tags disappeared from all three of my wordpress blogs this morning (all on wordriot.org domain). All three sites are running different plug-ins so I don't think it's plug-in related.
I tried this fix out to correct the missing descriptions in the wp_term_taxonomy table of my database: UPDATE wp_term_taxonomy,wp_terms SET wp_term_taxonomy.description = wp_terms.name WHERE wp_term_taxonomy.term_id = wp_terms.term_id
While that took care of any missing data in the database, when I log-in to WordPress and go to Categories, none display, and if I attempt to add a new category, it too disappears. Under Post Tags, the tag cloud appears and the number of entries associated with each individual tag appears, but when I click on the number of posts associated with the tag, I get No posts found.