Hi plex, the message you got should have a link to the original torrent page. Click on this and you get to that page. Just download the torrent again.
I assume you still have the file you downloaded - if you haven't you won't be able to reseed.
Open the torrent file again as usual, but when it asks where you want to save the file, point the torrent client to the same file you originally downloaded. The torrent client should say 'checking' and when it has checked say 'Seeding' or 'Queue seeding'.
And that's it.
If it says 'downloading' it means you haven't got the right file or you haven't got the complete contents of the file. Just stop the download.
🙂