If your “WordPress Categories Mysteriously Disappeared” the problem is most likely related to permissions to the “/tmp” directory.
Here are the steps to fix the issue:
1. Try to access your mysql server with phpmyadmin ( or you can Shell into your server, if you do not have phpmyadmin installed)
2. With phpmyadmin, if you try to access the “wp_postdata” or the “wp_term_taxonomy” you will likely get an error message such as :
MySQL said: Documentation #1 - Can't create/write to file '/tmp/#sql_67e9_0.MYI' (Errcode: 13)
4. If you attempt to access the “/tmp” folder using shell or ftp you will get a permission denied.
Unable to access the tmp folder (permission denied)
5. If you have control over your server you can fix this yourself (chmod, flush tmp folder …) otherwise You will need to contact your Hosting provider ASAP and have them fix the permissions to the “/tmp” directory.
(you can use chmod and other linux tools to help resolve you this issue, but i found out that most of them do not work in hosting environment since they may not allow you to tamper the permission on the “tmp folder” for security reasons)
Also if your /tmp Folder is hosted on a different partition you may need to double check to see if you are not running out of space on that partition. In such case you will need to empty out the /tmp folder.