Once a record is deleted, it cannot be "undeleted". This is why some developers design their systems so that records cannot be deleted. Instead, "deleting" a record marks it as "deleted" by entering a value in a specific "status" field. The interface is then designed to hide such "deleted" records from the user. That way, a record can be recovered simply by changing the status field so that the record is no longer marked as deleted.
In your case, if your table has a primary key field where you can open the back up file and add an occurrence of this table from the current copy of the file and link it by the primary key field. You can then find all records that do not have a related record in this table to pull up a found set of all records that were deleted from your current file after the backup copy was made. You can then import this found set back into your current copy of the file.