I am running Ubuntu 12.04.3 on a 64-bit machine with an nvidia graphics card.
Today I installed updates which included updates to one of the Nvidia packages. After it got done building the new modules with dkms I logged out but could not get the greet. So I rebooted and was thrown to a command line. If I tried to restart lightdm from /etc/init.d/lightdm I was just shown the boot logging screen. If I tried to run start lightmd I just got "restart: Unknown instance:"
So I uninstalled nvidia-* and then installed nvidia-common nvidia-settings and nvidia-304. Finally I ran sudo nvidia-xconfig. After it installed and I rebooted I can now get the lightdm greet. However, no matter which desktop I choose the screen just goes black and then brings me back to the greeter. I have tried Unity, Unity-2D, Gnome, and Gnome Classic (No Effects) all with the same result.
I do not see any problems reported in /var/log/lightdm/lightdm.log, however /var/log/lightdm/x-0-greeter-log does show the following, which I am not 100% sure is relevant.
[+0.96s] CRITICAL: ido_calendar_menu_item_set_date: assertaion 'IDO_IS_CALENDAR_MENU_ITEM(Menuitem)' failed
