I've just upgraded to 11.10 and now nvidia-settings won't launch. If I run it from the command line (as me or as root) it just hangs.
How can I debug this?
Try to completely remove your nvidia drivers from your system:
sudo apt-get purge nvidia*
Remove your xorg.conf
sudo rm /etc/X11/xorg.conf
Reinstall xorg completely
sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx:i386 libgl1-mesa-dri:i386 libgl1-mesa-glx:amd64 libgl1-mesa-dri:amd64
sudo dpkg-reconfigure xserver-xorg
You should be greeted with lightdm, this will default everything x the same way a fresh install would. After that you can try installing nvidia from the "Additional Drivers" application.