This may be the video card.
To install it, you must go into the Recovery Mode. To do that, hold SHIFT at startup so the GRUB menu is displayed, choose Ubuntu Recovery Mode after that. Choose to mount the disk as read/write, then drop to the root shell with networking (this is very important).
Then do the following commands once you're dropped to the root shell:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install fglrx
sudo aticonfig --initial
sudo reboot