So with LW 4.18 you used the Resume function in the library window? That's what you were supposed to do. Go to Library, click on incomplete folder, select all the incomplete files & press the Resume button. I don't see you having tried that. You said no incomplete files show up at all. I doubt that because the incomplete folder reflects what files are in that folder, 'if' you are downloading to the same download folder of course so LW is looking at the appropriate incomplete folder location where the incomplete files actually reside.

At present LW 5 has no technique of recognising/resuming incomplete files if they do not show up as unfinished downloads.