Begin in the PetExec menu under Owners & Pets-->Undelete Owner. Search for the owner to "undelete." You can search for an owner by first name, last name, phone numbers or email address. Or click on the blank search box to bring up a list of all deleted owners.
Click the "Undelete" link to restore the owner to active status. Once you accept the dialog box to go ahead, the owner is undeleted, the owner and related pets will be reactivated and you will be redirected to the Modify Owner screen to make any updates.