Is it a gnutella network file or torrent file? If a torrent file not a lot you can do except hope someone comes online with same file as more of the file than you do. For example, a torrent file I have has been stuck at around 40% for 3 months because nobody else has more of the file than anybody else.
If it's a gnutella network file, try this: delete your LW Preferences folder whilst LW is closed; instructions here: Point #3 in Fixes for Limewire (sample image included 2nd last post of page.) Make sure you back up your download.dat file. The problem might be mucked up LW preferences.