The download is automatically resumed if the source host is still available and you did not kill the download. However you can resume any file that is still in your incomplete folder. You have to find the same exact file again - the file size is very important unfortunately it is hard to figure out what the initial file size was since your download was broken. you might have to try some search results.

If you still have the search result available you can just download it from another source if there are multiple sources available.

mahalo