12.04 GPU lockup
Install the Nvidia proprietary driver. In a console do:
sudo jockey-text -e xorg:nvidia_current
Then reboot.
You may try using the nouveau driver, using sudo apt-get remove nvidia-current, followed by sudo apt-get install nouveau If you are currently using nouveau, remove it by running sudo apt-get remove nouveau, and install nvidia-common, assuming you have a running system installed and at least able to connect to the internet and get a wrking command-line. Once you remove and install the new driver, be sure to reboot.