You will have had an email warning that your forum was due to be deleted and another when it was including a link to download a backup file. These can be treated as spam by your email provider so you may have missed them.
Nabble only holds backups for a few days, so if deleted a while ago you may be out of luck. I have more information at:
http://greghelp.991552.n3.nabble.com/Restoring-A-Deleted-Forum-td4012788.html
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.