Solutions:
- Restarting the database sometimes fixes it
Solutions 2:
The /tmp folder on the server is filled up. When that's the case, the categories dissapear in wordpress. If you have root access to the server, then type:
df -h
This will give you the status of free space on your partitions. Make sure that /tmp isn't 100% used by deleting the files inside it. try these commands to empty the /tmp
Run:
rm -rf /tmp/* 1> /dev/null 2>&1