Do you mean the list of files you have downloaded that show in the Download Manager or the actual files you saved to the hard drive ?

Downloads are considered history items: clearing history removes corresponding download items and vice versa. You would need a backup copy of places.sqlite to restore deleted history. If files you have downloaded are lost then you will have to download them again if they aren't in the recycle bin, you can possibly check the history if you haven't deleted that as well.