This 'blog has been bit hard by a bug in a plugin.
WP-Sweep includes an ostensible ability to purge categories and tags that are not used for any entries. Unfortunately, the programmer made an error such that the categories and tags purged are any not used in generally accessible entries; tags and categories used exclusively in restricted entries are therefore purged.
Still more unfortunately, I did not discover the bug until after I had made various content changes; simply restoring a previous version of the database would undo those changes. I don't remember what they were, and my only records are the database and saved versions thereöf.
I will, over time, try to repair the damage.