Fatal Error, or stuck at "Done Loading"

Whenever I log in, and hit "Update Now", it starts loading for about 1 second. It then displays "Failed to Launch. A fatal error has occurred (4): null". If I do not update to 1.5.2, the game loads, but gets stuck at Done Loading and goes no further. I have tried to download and re-download, along with deleting all of the files, but nothing works. I have also tried updating my Java version.