See this article: Where is the Registrant List located?

Click the View Deleted Registrants button

Deleted registrants are permanent.  Deletions cannot be reversed because there may have been sessions and sales items associated with their registration and their selected items may no longer be available.

If you need to "restore" a registrant, use the information from this screen (by printing (1) or downloading (2) this information) and re-enter them as first-time registrants.

View the Change Log (3) for deleted registrants to find out who deleted them.