Go to LW's library, click on Incomplete Files. Highlight the file & then click Resume. The file will go back into LW's download pane

Files will show 0% until they find sources, then the download % should pick up from where it left off. If files are stuck, start a new search for the file name - sometimes that can kickstart a download. Also, once you've started a new search & see the file that you're downloading in the search results (shows a folder with a green tick), you can right click on that > search more > get more results.