sudo apt-get --reinstall install nvidia-currentIf this does not work, you might also try
sudo apt-get --reinstall install lightdmA few days ago, Ubuntu updated its kernel. The lightdm loaded but the configuration was wrong. I realized I had to re-enable the additional drivers. A little surprise was the "version current" stopped working, while the "post-release update" worked. The other thing annoying me was the blue face problem of youtube flash video. The fix was described in this post. The most tricky part was that you can only turn off the hardware acceleration option when the video was in full-screen mode.