shell disappeared randomly and system freezes; what happened?

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http://askubuntu.com – So I was using the GNOME-shell one day. It was running with no problems for about 3 hours, then the top bar and the window borders randomly disappeared. I was still able to see the content inside the windows, and for a while, I could drag them around, too. I couldn't access the run prompt via Alt-F2 to reset the shell, and trying to open the Terminal (Ctrl-Alt-T) didn't work, either, so I figured that keyboard shortcuts were no longer working. So then I opened Nautilus by clicking on the Home icon on the desktop, which still seemed to work, navigated to /usr/share/applications, and opened th (HowTos)