If it is the case that the forum has not been used for a long period then under Nabble's terms and conditions the forum will be deleted.
They do send an email to warn you in advance of this, but it is possible it has been treated as spam by either your mail service or your own software. This is why Pedro asked his question.
If the forum has been deleted there will be no backup held by Nabble. Had you chosen to select OPTIONS > DOWNLOAD BACKUP while the forum was active, then you would hold a file and Nabble could upload it to a new forum - but I'm guessing that you did not do this.
Sorry to be the bearer of bad news.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.