Can't boot 14.04

[Ralf Mardorf]

One other possibility would be to downgrade, if the old packages should
be still available by the cache.

  cd /var/cache/apt/archives/
  ls -hAl nvidia-304*131*deb nvidia-opencl-icd-304*131*deb nvidia-settings_331*deb

If so, then run

  sudo apt-get purge $(dpkg -l nvidia* | grep ii | cut -d" " -f3)
  sudo apt-get update
  sudo apt --no-install-recommends install nvidia-304*131*deb nvidia-opencl-icd-304*131*deb nvidia-settings_331*deb

After that you need to hold/lock the packages.

http://askubuntu.com/questions/18654/how-to-prevent-updating-of-a-specific-package

Regards,
Ralf