This happens when an upgrade is done without removing the non-free graphical drivers, normally Nvidia or ATI cards.
Actually before installing you should have disable those drivers, so its crashed now.
To fix you have three options :
1. To boot from live cd (karmic), backup what data you want and do a fresh install.
2. Follow this, open your terminal after booting from live cd and then :
type this cat /etc/fstab | grep "/ was on"
the o/p will be like : # / was on /dev/sda1 during installation