If you've been able to download part of the file & are sure the file's not a fake (you've been given the info about fake files before)...right click on the file within search results > search more > get more results. Or start a completely new search for the file, sometimes that can give the download a kick along.

If it's a rare file with few sources, maybe the sources have gone offline. In that case, keep searching at different times of day...people are logging on & off all the time, so available files are constantly changing.