Last night, one of the office PCs tried to install the Win10 Anniversary Update via Automatic Updates at 3:30 AM or thereabouts. By 8 AM,
the black screen with spinning balls (no message) was on the screen and was there til 9 AM when the PC was shut down via the power button.
After bringing the PC back (Restoring the previous Windows version) following a few more hard shut-downs to the previous build, the Anniversary Update was forced by going to the Microsoft page and downloading the update - and then running it.
After requesting to Update Now, the process ran through up to about 79% before the screen went black and the PC rebooted. It looked like it would boot all the way to the Desktop; the Windows Logo appeared on the black screen along with the spinning balls (no message). But soon after the Logo disappeared, leaving only the black screen with the spinning balls (AND NO MESSAGE).
The HDD LED on the case stopped flickering and trying Ctrl-Alt-Del does not do anything. See the picture, above.