sudo /etc/init.d/gdm start
If you don't get the GUI, try Ctrl+Alt+F7. If that doesn't work do Ctrl+Alt+F1 and enter the code:
sudo dpkg-reconfigure xserver-xorg
This will start a text-based "wizard" that will ask you questions about your keyboard, your mouse, your graphics card, and your monitor. Answer the questions as best you can. If you don't know the answer to a question, just go with the default and press Enter.
When you're done, press Control-Alt-F7 to get back to graphical mode and then Control-Alt-Backspace to reset the X server (so your changes can take effect).
The above thanks to: http://psychocats.net/ubuntu/nox