Ok in a perfect world you have a backup of your SQL DB. But last time this happened my DB was less then 24 hours old, but I had put in 5+ hours into WP! So I followed some troubleshooting steps but eventually decided I wanted to try and reinstall MySQL but without losing my DB's. Wasn't sure it was possible. Well it is!
First off, I would make a backup with CloneZilla.
Try and Reboot to see if that fixes.
Ok lets reinstall MySQL and Backup DB's.
Now check /var/lib/mysql. If the files are still there then I would just fix up permissions and re-install MySQL.
If not Copy them Back via: