Contact the Helpdesk and indicate the link to that torrent. If it is not seed at all, a staff member will try to revive it with the usual steps (reseed request, promotion, comment, …). If that has no success after a week, the torrent will be deleted and the way for your re upload free.

With good torrent clients (e.g. µTorrent, BitTorrent, Vuze, ... it is possible to jump on a torrent and help seeding only the files one got. That isn't a true reseed though. It has the disadvantage tol be count as leeched torrent by the tracker and there is a limit in number of leeching torrents. Upload is correctly added to your stats, no download should be made.