not able to boot ubuntu 11.04 graphical anymore

HTML Code:
sudo apt-get update
results into
HTML Code:
W: Failed to fetch http://ppa.launchpad.net/unity-2d/ubuntudists/natty/main/source/Sources 404 Not Found
W: Failed to fetch http://ppa.launchpad.net/unity-2d/ubuntudists/natty/main/binary-i386/Packages 404 Not Found
Did not find anything suspicious in the log-files in /var/log

Probably something messed up in sources.list, etc...?