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

its /dev/sda1 for me might be different for you, not that down

Assuming its /dev/sda1 the next steps are giving below, replace /dev/sda1 with yours.

{Copy paste each line or the whole set however you can, to the terminal}

<b>
sudo mkdir /media/karmic
sudo mount /dev/sda2 /media/karmic
sudo mount -o bind /proc /media/karmic/proc
sudo mount -o bind /dev /media/karmic/dev/
sudo cp /etc/resolv.conf /media/karmic/etc/resolv.conf
sudo chroot /media/karmic apt-get update
sudo chroot /media/karmic apt-get upgrade
sudo chroot /media/karmic apt-get dist-upgrade
</b>

3.Drop in the recovery shell with networking and then :

<b>
sudo apt-get install xorg-driver-fglrx
sudo aticonfig --initial
startx
 
Again from terminal after logging in 
sudo apt-get update
suao apt-get upgrade
</b>
<code>
 
ALL THE BEST :O)