Suddenly, yesterday, all my categories and tags disappeared from my blog. It was as if I had never tagged any posts, and never used any categories. I couldn't even see the default category in the admin panel!
Here's what went wrong.
I hadn't realized it, but I was running out of disk quotas. Actually, I had no space left on disk on my server! This caused some trouble to mysql (obviously).
All I had to do was to clear up some disk space (I deleted some big files, and backup folders), and restart the mysql service.
Dada! Everything works fine now.