I'm guessing this is related to ACPI not working as it should.
Try enforcing the power interface as a boot option to see if this fixes it.
First open a terminal and try
sudo init 0
and see if it turns off properly (i'm guessing it won't, but just to make sure).
If it does not, reboot, when grub appears press e, this should show your boot parameters
linux /boot/vmlinuz-linux root=UUID=<something-something> quiet splash
add acpi=force to it
linux /boot/vmlinuz-linux root=UUID=<something-something> quiet splash acpi=force
hit enter to boot, now try to shut it down again and see if it turns off.
If it does, you can make this option permanent by editing the grub config.
Make a backup of your current grub.cfg (just in case)
sudo cp /etc/default/grub /etc/default/grub.backup
Now open the file in an editor, like
sudo nano /etc/default/grub
And search for
and change it to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force"
save it, and run
and try shutting down again. This is the same as before, but permanently added, if you want to go back/restore your backup, remember to run update-grub again, after changing the config.