If you catch this quickly enough, you can use the Undo button in the iPad app’s toolbar to undo the delete. The toolbar is at the top of the screen, and the Undo icon is near the left and looks like an arrow which started pointing right but turns back around to point left.
If you didn’t catch this quickly enough… are you syncing with a Mac by any chance? The Mac makes regular backup copies of your database which you can access from File > Show Backups.
If you’re not syncing with a Mac, are you using our Omni Sync Server? If so, we might be able to retrieve your data from a backup: please contact omnifocus@omnigroup.com and let us know your sync server account name and the approximate date/time at which you deleted the folder.
But again, ideally you would just press the Undo button. Hope this reaches you in time!