If you deleted the message, then it is gone. But Nabble has different meanings for "delete" and "remove".
Delete means physical deletion and there is no way to bring the message back, unless you have saved a copy of your forum using
our backup feature.
Remove means that the message was moved out of the forum, but still exists on the author's profile page. So you may look at the profile of whoever wrote that message to see if you can find it (click on the avatar/user name of that user and you will see his/her profile).