Xorg server configuration fail - no screens detected
Solution 1:
OK, so I have reinstalled the xserver-xorg with apt-get and then removed the ~/.config/monitors.xml file and everything works fine now. :)
( The monitors file is like an artifact after connecting an external display and it stores the settings somehow and causes problems when you switch back to one display mode http://ubuntuforums.org/showthread.php?t=1917220 )