How can I leave tty?
Normally, can enter a tty with Ctrl+Alt+(F1-F12), and in Ubuntu, and can exit it with Ctrl+Alt+F7. This problem is probably due to the login manager not starting. Try:
sudo service lightdm start
for the default lightdm login manager, and this for Gnome's login manager:
sudo /etc/init.d/gdm start
You can also try entering the user session directly:
startx
Pleases note that some of the above may be guesswork...