Our Web  page has been hacked last week ! We were still using and old 2.5 Joomla! version that was not supported anymore since dec. 2014... The hackers  certainly used a security hole to hack our site. The site was only displaying a blank page.
Fortunately, i'm making regular backups and started a few months ago to convert the site into the latest Joomla! version.
So it didn"t took me to long to bring the site back online. There are still some things to do, though. In particular, the language translations...
FaLang translation system by Faboba