Sucks. I've never used Ubuntu but lets have a crack anyway. Alt-F2 to get to vt/2 and use the following commands to check some stuff.
First, use this to read through the Xorg log and see if anything jumps out at you as a problem. If not, try the list below, again just see if anything jumps out as wrong.
# check that nvidia is actually in xorg.conf (basic but never hurts)
grep nvidia /etc/X11/xorg.conf
# check the nvidia kernel module is loaded
lsmod | grep nvidia
# check all the nvidia symlinks are correct
# they should point to libGL.so.1.0.8178, same with libGLcore
ls -l /usr/lib | grep GL