On 24 November 2016, I was using Windows 10 with Norton Utilities, both with all the latest updates.
I ran 'Repair Drive' on the system (C:) drive. The PC rebooted and some information was then shown as Norton did its work.
[At this point, while Norton was working, I may have pressed a key - either the space bar or one of the keys on the bottom row - this almost certainly was not related to the problem].
The PC rebooted again and then the message 'Wait for Device Ready' was displayed for a long time. I left it overnight. When I returned in the morning, the screen was blank and the PC did not respond to any keys or mouse movement, so I had to turn it off and reboot.
From then on, whenever I rebooted the PC, the following messages were shown:
"Wait for Device Ready
Norton Disk Doctor (followed by some asterisks)
Your PC ran into a problem"...followed by Windows options to try to solve the problem.
The PC would not progress beyond this point.
None of the Windows options solved the problem...I have worked in IT for 20 years and I am sure I tried them all.
So my only option was to go back to an old System Image I had of the C drive.