Sgt_Kickaxe - 1:32 am on Nov 1, 2012 (gmt 0)
This seems to be something that many webmasters will eventually face to some degree and all should prepare for, I feel for you because I know how much it stinks. You're doing what you can so... some other ideas:
Longshot: if your site was as large as it sounds contact Google/Bing/Yahoo to see if they can provide a static copy of the pages they knew about and get those back online, if they can provide them. I realize some functions will no longer work but it's best to maintain rankings for existing content if at all possible which won't happen if the pages remain down too long. Google/Bing/Yahoo is not in the business of providing such a service but on a larger site with solid content and thousands of users they *might* help.
Immediate: Quarantine all computers used by your team if it's in an office setting, obviously you can't do the same for home computers without court order. Someone is responsible and, unfortunately, the culprit is more likely than not to be someone in-house. Forensic evaluation might turn up clues but even if it doesn't you need to know, now.
Contact your CDN provider, if you had one, and ask about retrieving a backup copy of what they had.
Core code - this should absolutely have been backed up to an offline computer or drive as this is the heart and soul of any website. Your developers should each have had a copy or access to a dev server with a copy as well. The total size of the site may be terabytes but the core code would be much, much smaller. Contact your web developers and hope one of them still has a copy.
edit: if this was a registered business with employees in an office setting I would contact police. Even if the police can't help, you need to take care of what you can from a legal standpoint. You don't know how things will play out yet but they may very well end up with you figuring out who did it and seeking damages in court and it's on you to have done what you could.
Not having a backup will work against you in court as it would have minimized damages though... but still. Good luck.