It may also depend on the tracking server. Many torrents use The Pirate Bay as a tracker (even if you didn't find the torrent on the pirate bay). Due to legal issues, The Pirate Bay's tracker went down a few days ago and has not come back up. However their website remains up... so you can see the merchandise... you just can't touch it.

Check what the tracker URL is on the torrent you are trying to download. If it is the Pirate Bay, you have found your problem.
You may have some other problem, but that is the first thing that came to my head.