If you don't know which download you cancelled in error, and you have a pile of files in your incomplete folder like I do, you will have to try trial and error, I'm afraid!
If the search window you started the download from is still active you can click the column with the stars and at one end of the column you should see a broken icon. If you download that ... hopefully it will be the one you cancelled and will continue from where it left off.
If you cancel a download, you need to clear it from the download window. Once you do this you should see the broken icon I mentioned before, and you should be able to continue the download.
It usually works like this
I hope this helps