Variables are not actually deleted from your data file, but are removed from the Variable Index (until you undelete them).
Variables can be deleted by using the Variables | Delete variable menu item. You can delete any variable (including those in the original data file).
Variables can be undeleted by using the Variables | Undelete variables menu item. You will see a list of all the variables that have ever been deleted. Select as many as you wish, and press OK to undelete.