I just experienced this bug on an upgrade from Xubuntu 13.10 to 14.04 with the proprietary Nvidia drivers.
/etc/lightdm/lightdm.conf had both greeter-session and user-session defined as 'null' (user-session=). Manually setting the right values fixed the issue. ** Changed in: nvidia-prime (Ubuntu) Status: Fix Released => New -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to nvidia-prime in Ubuntu. https://bugs.launchpad.net/bugs/1267442 Title: Install nvidia-331 on X/K/Lubuntu results in unbootable machine Status in “nvidia-prime” package in Ubuntu: New Bug description: Install of nvidia-331 completes, assuming it edits lightdm.conf left with this - not any use if you're not using Ubuntu greeter-session=unity-greeter user-session=ubuntu display-setup-script=/sbin/prime-offload display-stopped-script=/sbin/prime-switch have to comment out the exisiting greeter and user session lines and replace with display-setup-script=/sbin/prime-offload display-stopped-script=/sbin/prime-switch user-session=xubuntu greeter-session=lightdm-gtk-greeter to get a satisfactory boot To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/nvidia-prime/+bug/1267442/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp