Try deleting the file that you downloaded. It will be in the Jedi Outcast folder, where the "assets0.pk3" file is. Find the specific .pk3 file that you were downloading, move or delete it (do not delete ones named assets[number].pk3!), then try restarting JO. If you can't find the file or don't know the name, just remove all of the .pk3 files in the folder except for the "assets[x].pk3" ones. After you do that, you can test which file does not allow you to start up properly by putting them back one at a time.
"Words are deeds." - Wittgenstein