After the newest wordpress update 3.5, it automatically made a folder like this. I saw it when I logged into my hosting via ftp today. I hosted 2 sites (2 unique domains) there and both have wordpress 3.5, but only for one site, this type of folder created automatically. mysite.com.old Anyone experienced the same thing ?
Did you do any kind of back up of the site before you did your update? Could be a hacker, or something auto generated by your hosting provider if you did the update through a one click installer.
I had a few problems and ended up having to downgrade from Wordpress 3.5. I think WP 3.5 was released with some glitches.