Mac (Retina 5K, 27-inch, Late 2015) when shut down often goes into slumber mode and will not wake up. I then have to switch it off from the off button. And sometimes goes into this coma mode when I switch the computer on.
There is no "slumber mode" or "coma mode". When you shutdown it powers off the computer. There is no way to "wake it" if it is powered off. You have to power it on using the power switch in the back of your iMac.
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.
Try resetting the SMC… Resetting the System Management Controller (SMC) on your Mac - Apple Support
If that doesn't work… Using Apple Diagnostics - Apple Support
Mac (Retina 5K, 27-inch, Late 2015) when shut down goes into slumber mode and will not wake up
