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 when 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 linux and dont know what to do.