Let me see if I got this right, you moved your forum to a new domain, fully intending to place 301 Redirects in place. But the owner of the old domain, that hosted the sub-domain you used, deleted your account there. This means all of your old backlinks now point to non-existent URLs.
You have effectively lost all of your old backlinks and now you are ranked much lower than you were before you changed domains.
All you can do at this point is try to contact as many of those old link sources you can find and ask them to update the URLs to your new domain. You are essentially starting over from scratch. What you need now is some heavy duty promotional activity.
Try articles, press releases, blogging, guest blogging, advertising and every promotional activity that you can think of to get the traffic flowing again.