When clicking the UNDELETE link on the tracking page, you'll be taken to a page like the one shown in the screenshot to the right. It lists all submissions that have been deleted from the form. It's not the most usable page ever, but it does include a search option to let you search the deleted submissions. This will hopefully help you locate the one(s) you wish to undelete. Just click on the VIEW link to view the details of the submission. Once you've found the one you need to undelete, you can undelete it on that page.