I was facing this issue and had no clue how to resolve it. After playing around with grub and various lightdm settings and reading bunch of forums related to the problem I could not resolve it.
I thought it was a longshot but just tried to reinstall the nvidia-drivers and Voilla!! So, for people who faced similar issue, make sure you have the Nvidia driver in one of the folders and boot into text mode(check ubuntu forums on how to modify grub to boot into text mode ) and then reinstall the drivers.
Hope this solves your problem :-)