My computter wont shutdown

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http://askubuntu.com – If I try to shutdown my computer it simply takes me back to the login-screen. I looked around and found : Type in terminal: 1. sudo gedit /etc/default/grub 2. Find the line: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" 3. Change this to: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force" 4. Save the file and close the file. 5. Finally, in terminal: sudo update-grub but now there is a new problem. if I try step 5 it says: myname@ubuntu:~$ sudo update-grub [sudo] password for myname: /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?). myname@ubuntu:~$ I am new whit lin (HowTos)