Just an update. I've fixed the issue.
After removing the battery, CMOS battery and RAM, and leaving overnight, I was able to get the system working again.
I rolled back Windows to the previous version (OS build 16299.15). Everything in my computer is fine...I put everything back in it's place, and all was good.
Just to make things safe, I've got rid of the setting where the computer would go to sleep if I closed the lid. It now shuts down instead. And I also set the computer to never sleep.
As expected, Windows decided it should update (because the update had been downloaded), so I let it. So now I'm back to the 'problematic' OS build 16299.64
But so far, because my computer will never sleep, it has been ok.