How to fix sudden Ubuntu 14.04 graphics problem?

I have a dual boot Lenovo laptop (Windows 8 / Ubuntu 14.04) which worked just fine. Until today. When I log in to Ubuntu I type my password as usual and select for example 'Ubuntu' as the environment. After that, I only see the background image and a mouse pointer I can move, but nothing else. No menu, no bar, nothing! Also when I choose GNOME Flashback (Compiz) or GNOME Flashback (Metacity) the same happens. When I choose KDE I am able to see some bars and menus, but unable to click anything!!

How can I at least start in a terminal mode to save all data and reinstall Ubuntu from scratch)?

To start again I always need to power off the laptop by force (i.e. pressing the power button).

Addendum:

• I tried to start in recovery mode with failsafe graphics mode. Here also nothing starts, even after waiting for minutes.


• I have not changed any hardware.


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  Yesterday I tried to connect the laptop to an external monitor via HDMI without success. I did not change any xorg configuration.

  
  


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  Attempts to fix this problem made it only worse. I backup all data and re-install Ubuntu 14.04 from scratch.