Posting here is the appropriate action to contact one of the Nabble team.
However, Nabble does not keep backups of individual forums, only of entire servers, to use to restore them should they crash, and these are replaced and overwritten regularly, at least within 24 hours.
As I understand it, therefore, unless you can provide a file generated via "Options > Download back up" then a deleted forum cannot be recovered.
Without much optimism, all I can suggest is that you post the URL of the forum in case it can still be recovered.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.