That is absolutely astonishing, and, quite frankly, completely unacceptable. If the administrators of this site are so incompetent that they can't even be bothered to occasionally make a backup of the site, we should not be wasting our time commenting on it. Computers crash or get hacked all the time, or things get wiped out by human error (as appears to have happened here), and it would be a travesty if that were all lost because of stupidity. We should start a new site that is actually backed up, rather than relying on irresponsible administrators.
Perhaps the site admins should make a complete copy of the site available as a torrent that we can all download, to distribute the backup around the world, so in a worst case scenario someone else could bring it online again. That's what I do with my most important (tens of thousands of unique visitors per week) web site -- if I get hit by a bus, I don't want all that user-created content to vanish.
In the meantime, someone should volunteer (preferably the administrator responsible for wiping out your content) to go through archive.org to find all posts by you that were archived there, and manually load them back into the forum with the right dates and everything.
This whole thing is making me reconsider participating on this forum, unless the administrators can apologize and explain the actions they are taking to make sure nothing like this ever happens again. Back in the days of Usenet we never had this problem, but now with forums being centralized under individual control it makes us realize how much better things were in the 80s and 90s.