Google suggests this might be a "black screen of death" issue, but the descriptions are diverse and the dates often seem to relate to issues from 2013-2014....
Prequel: About 3 days ago I tried to update my dad's PC antivirus (kaspersky 2016). Short version, I got safe mode flashing/blinking but no actual login, and had to run back through the boot menu (which I'd set up ages ago) to use "Last good config" and system restore, which fixed it.
However, today for the first time in some months I tried to use my own laptop (Dell). Its a completely different system, also Win 8.1. When Id finished with my work, as usual I opened win updates and updated (about 12 updates I guess from memory?); got lots of fails, not too bothered, rebooted with plans to enter safe mode and fix (by clearing WU caches if needed) after the reboot. The laptop has been 100% stable and reliable since being installed over 2 years ago, but sometimes WU needs a bit of a clearout, no big deal I figured.
On reboot I entered safe mode, but instead of safe mode, I got the identical flashing safe mode as the desktop had got earlier, with no usable UI. But unlike with the desktop, after trying to fix I'm now stuck.
I can get into bios, but either F8/shift-F8 isn't working, or the black screen is blanking it out, or I'm constantly unable to get in there fast enough.
Whichever it is, I'm 100% sure from the circumstances that its some driver or update issue. But all I can get is bios logo, a half second Win 8.1 bootup "spinner" and then a completely black screen.
There's definitely life under it because caps lock still works (light on/off) and after a few minutes idle the black screen is visibly slightly dimmed, but ctrl-alt-del, win-u, ctrl-shift-escape etc, no keyboard action seems to help, and I can't seem to get to any UI that lets me try anything else.
Things I haven't yet tried - remote access (no suitable 2nd PC and from memory remoting in is probably disabled on principle), 2nd external screen (none here and FN+video.switch key does nothing), bios (nothing blatantly helpful?), recovery system via bootable windows DVD (none available till Monday).
I've read several threads, they all seem to be slightly different - older issues, win 8 upgrade, safemode or boot menu obtained, flashing rather than fully blank, or whatever. So this seems a little different from those.
Any ideas beyond "back up data and reinstall"? Especially, as its now happened on 2 very different systems in as many days, is there a known cause or fix, or some WU or action triggering this to avoid?