Don’t worry, you can resume the download as long as you don’t delete the downloaded file from the file manager.
To resume the download, add the magnet or .torrent file again and once you add the torrent, just long-press on it and from the context menu, select ‘Force Recheck’ option. By using ‘Force Recheck’ option, the torrent will check for the partially downloaded file and resume the download from where it left off. Please see the image below from 
Torrent Pro.