Select the torrent that has this error.
Right click on it.
Select Set Download Location.
Choose the folder that you downloaded to.
By default it should be the Downloads folder. You have to select the folder not actually be in the folder itself. It should bring up by default the Downloads folder, but you have to go back and select the folder itself. Hopefully you know what I mean.
Right click on the torrent.
It should start back up again.