It depends on what you downloaded it with. If it was via torrent then you might not have any luck in salvaging it. (BT doesn't pull the file from beg to end, it pulls little file segments here and there and then puts it together)

If you got it from an ftp server or *evil* filesharing program (which neither I or techzonez support in any way) then you can try playing it in avipreview.
http://www.avipreview.com/download.htm