this has happened many times to my almost finished movies
like around 500 or 600 mb they disappeared
Also you might run out of space on your HDD.Originally posted by From FAQ
When your computer crashes or shuts down due to power failure, it can happen that some of your partial downloads (.dat files) get corrupted. If the part where all the essential details of the file (name, hash, checksum, size, sources) are stored gets messed up, then Kazaa Lite is not able to resume the file. Once Kazaa Lite encounters such a broken file, it will delete the file.
Crashes are often hardware related (wrong/old drivers, incompatibilities between hardware). So make sure all your drivers are up-to-date. Also keep your software up-to-date. A stable Operating System such as Windows 2000 or XP can help a lot to prevent crashes.
Otherwise there is no other solution than making backups of big dat files once in a while, so that you don't have to start the download all over again
As per Autumn Fox most common cause of this is corrupt files. If these are movies you should be checking with AVI preview or Videolan at beginning of download. If you get nothing (blank screen) then there's a good chance its corrupt. Use the verifieds from Movieworld for links to good copies.