Q: Mac (Retina 5K, 27-inch, Late 2015) when shut down goes into slumber mode and will not wake up

I did not explain myself well.  When I select shutdown the screen goes very dark blue and I can see the curser and it remains like this.  There is a very faint hum that tells me the computer has not shut down.  I leave it for at least 10 minutes but it is still the same and using mouse or keyboard makes no difference.  Then I use force quit via the shutdown button at the back of the display.  The screen now goes black and there is no faint hum.  I hope that explains it better.