Can't shut down MacBook
You can still reboot your Mac even if the Shut Down prompt never appears: holding down the power button for at least 30 seconds will cause your Mac to force shut down, then releasing the power button and pressing and holding again until the Apple logo appears will allow you to start it up again.

If your Mac doesn't have a power button key, you can force  a shut down by pressing and holding control + option + command + eject .

Source and more info here.

tl;dr: Hold the power button for a while!