In my case these simple steps worked for me fine:

1. Press Ctrl+Alt+T to go to a terminal and type:

   sudo gedit /etc/default/grub
   

   This will open the grub config file. Change the line:

   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
   

   to:

   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force apm=power_off"
   

   Then save & close it.

2. Type in the same terminal:

   sudo gedit /etc/modules
   

   This will open the modules config, just add the following:

   apm power_off=1
   

   and save the file.

3. Next type in the terminal:

   sudo update-grub