I host multiple domains with this very popular hosting provider and I just went into one of my sites and... I see a black page with message "Hacked by...". I checked and all my sites with the provider are showing this same page.

Inside of file system I have seen the hacker placed all default.* and index.* files with this message. So the hacker overwrote all index pages, placed new pages and that is under every, I say again, every folder. Cleaning this up will be close to a most horrible job.

What to do (right now I am awaiting the restore of files from hosting provider)? How to prevent this? Whom to blame?