Go back to the tracker you got the file from and re-download the torrent. Make sure your client is pointing to the correct location where the file is stored. [set location, verify local data] and it should just seed it.
You can create a new torrent for that data if you want, that way you can continue seeding it in a new torrent swarm.
Otherwise if you intend to seed in the old torrent swarm then you need the old .torrent for that. Search around maybe you'll find it (or if you remember where you downloaded it originally that's even easier).
Depending on your client, there may be different directories for complete and incomplete torrents, so you may have to put the downloaded files in the incomplete folder. If your client only checks the incomplete folder when starting a torrent.