Windows 10 blank screen after BIOS boot stage, even in safe mode
Solution 1:
For the record I tried booting off an Ubuntu USB drive: worked fine of course but then all the Windows recovery console operations did also, there is nothing wrong with the screen, it was simply getting to the Windows log stage that turned the screen black. There were no restore points to restore to so I used DISM to determine the last update (which occurred the evening of the problem) and then used DISM to remove all of the packages of that except that it refused to remove the security update, which I guess was intended.
But that made no difference. I tried a Windows reset from the local Windows installation and that failed also, as did SFC /scannow, though chkdsk was perfectly happy with the disk.
I tried to reset Windows from a USB disk but that only works if you already have Windows running, which of course I did not. In the end the only way out was to reinstall with a network connection to drag in a new copy of Windows.
Just hope it doesn't happen again.