http://askubuntu.com/questions/168736/how-to-start-gui-from-command-line

Try to use startx in your tty , if it does not work on one terminal try ctrl+ alt + [F1 to F6] in order to switch a new tty

if you tried the command startx, but the error command not found was returned, then to install a graphical desktop, in case it is not installed, you would type this:

sudo apt-get install ubuntu-desktop