When something's pruned from the server, it's gone forever. The only way to bring it back would be to restore the forums to an older version backed up on one of our harddrives, so everyone would lose a month's worth of posts just to retrieve one message for you.

Sorry, but it isn't really a viable option.

If I'm not mistaken, Fuzz or Merlin mentioned something about moving any important messages out of the Inbox/Sent folders and into the Saved Folder (you can also create other folders if you don't care for the name) before the pruning occurred. Only messages in the Inbox and Sent folders were affected by the message prune. I know that doesn't help you retrieve any old messages, but it is something you should consider for the future.