Gnome-Shell vanished after System freeze: No panels, no focus, no hotkeys

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http://askubuntu.com – This system worked fine until now: Ubuntu 12.04, Gnome-Shell, Nvidia GTX 460 Then I installed a plugin for Firefox ( proxtube.com ) which required a restart of FF. I clicked 'restart now' and the system froze immediately (not even ttys were accessible) After a restart gnome-shell does not start correctly: There are no panels, menu-bars, or something (except the background and the desktop folders). No gnome hotkeys as well. Starting the terminal creates it, but if I start another program I can't focus on the terminal again. I tried replacing gnome-shell. I deactivated and later removed all th (HowTos)