If you have a broken display , rather a 640×480 display with no unity panels, you are one of the many who tried to install the new Nvidia beta driver which promised initial Optimus graphics card support for Linux.
In reality, the Nvidia beta driver 319.17 does not support the current Xorg nor the current Ubuntu kernel 3.8 series. Most of us had blindly installed the driver. To add insult to injury, it is not a straightforward uninstall of the Nvidia driver which can get you back a working desktop. The after-effects of installing the beta driver are no boot, no desktop, black screen, etc. In short, a completely unusable Ubuntu desktop.
However, the situation is not all that bad. You can still recover your desktop by performing a few steps. There is no need to reinstall Ubuntu.
1. Firstly remove the installed Nvidia driver
2. Restart your system after uninstall
3. Drop to console at login by pressing Ctrl+Alt+F1
4. Install bumblebee
sudo apt-add-repository ppa:bumblebee/stable
sudo apt-get update && sudo apt-get install bumblebee
5. move the config files from the home folder. Note: this may lead to losing stored preferences for your desktop and applications. you can copy them back one by one.
mv .config .config-bk
6. Restart your system
7. Login to get your old desktop back