The short story:
Check your CSM settings, that´s all I know.
The long story in the belief that it might help someone:
I have win8.1 64 eng. on a SSD disk.
Day 1, night:
-------------
Finished backup with robocopy to a NAS, got five disks. All is normal.
Day 2:
------
Started the PC the usual way. It got stuck after the win8 logo.
Black screen, cursor, no login, ctrl-shift-esc doesn´t work, ctrl-alt-del doesn´t work, no task manager.
Safe-mode doesn´t work, none of the commands you can do in CMD recovery accomplish anything.
System restore doesn´t work, automatic repair doesn´t work. WITAF (What In The Actual Fu*k)?
Booted with a live cd; all the files were there.
Hours of googling and testing. Got nowhere.
Day 3:
------
All right I give in, I do a refresh. So in addition to my aforementioned backup,
I copied all folders on c: to a USB disk.
Booted and chose refresh and got "harddrive locked". Great, another problem.
Googled a bit.
On day 2 I read about (U)EFI stuff and that lot, tested things and got nowhere. Probably due to this information
floating in my head I went in BIOS and checked the CSM setting more thoroughly. I run with CSM enabled.
I went in the submenu and changed "boot device control" from "UEFI and legacy OpROM" to "legacy OpROM only".
Restarted with the DVD just because it was still in the drive. Tried Automatic repair.
This time I got Choose a target OS.
LIFE! (in retrospect I might not have needed to troubleshoot at this point)
Automatic repair did some fixing and restarted.
Windows update appeared after win8 logo.
Installing updates 30%. "Failure configuring windows updates. undoing changes". Auto restart.
Got this update error looping. Great, another problem.
Let the PC just loop for itself while I googled a bit on my laptop.
After a few restarts it just fixed itself. And all is well.
Installed all windows updates, checked for system errors, restarted and all is still well.
- Did a windows update cause this? Don´t know. I run updates manually and last time was approx. a week ago.
- Was the BIOS setting changed somehow? Don´t know. I certainly didn´t do it.
--In short check your CSM settings, that´s all I know.
What Day 4 will bring is to early to say at this point in time...