It can be a lot of things as Buster has told us. But let's do the usual step by step first
1) Check what torrent program you are using. I prefer uTorrent the most but be up to date with it! Sometimes, some trackers detect older versions and block them due to activity in the past. If you're running an older version, update it ASAP!
2) Check that you are connected to your peers. I.e. do you see any seeds/leechers when you connect? If not, then you're not connected. If so, check your connection to the internet. Make sure your firewall is allowing the torrent program to have access and for god's sake, open the ports required for the torrent program. Default is usually 6881 - 6889 but you'd best open the default ports the program starts with and go up by 1000.
3) Check that the torrent isn't giving any errors. You can see that usually when the torrent checks for the tracker. Any messages of ERROR: XXXX or TIMED OUT means the torrent is dead.
