Can't boot 14.04

[Jim Byrnes]

This morning when I turned on my computer it booted to a guest session 
login in prompt on both monitors. When I entered my password the screen 
blanked for a short time and then the guest login prompt appeared again.

Software updater ran yesterday or the day before and I think one of the 
things updated was the nvidia driver so that seems to be the likely cause.

Right now I am booted to 12.04 on the same machine.  What do I need to 
do to fix the problem?

Regards,  Jim