This is sort of a bug report.

Installed Neon on a spare HD in August, set the desktop to look how I like but didn't install anything (other than an icon package), then it was put away and not used. Last week I decided to update and use it... it booted up fine, updated fine, restarted...

... and now I can't log in. Neither as user nor root. Doesn't accept any combination of root, user, and password. Says login incorrect, forever.

So I did the password reset thing as described on the Ubuntu help site, and that behaved normally, and wrote a new /etc/shadow.

Still can't log in. /etc/shadow looks okay internally, and is -rw-rw-rw- root root. However, the updated /etc/shadow has a new line at the end about coredump (it did not coredump, it shutdown and restarted normally after password reset, so I have no idea what that is about). Switched it back to the original /etc/shadow. Tried a known-good from another system. Tried deleting it (then it hangs at that point instead) Tried everything sane or crazy including strapping a black chicken to my monitor, and nothing worked.

Began to suspect it's hosed somewhere else entirely, and this is just the symptom. Found several complaints on reddit of black screen after latest update, which tends to support that notion.

I'm about to fling up my hands and reformat it, but is there anything else I should look at first, or extract for the devs' benefit?

I had a similar issue and managed to fix it.
I updated a new KDE Neon installation using the Discover app and could not login afterwards.
The login screen just looped as if my password was wrong.
Tried many things including creating a new user, with the same results.
Used Alt-Ctrl-F2 to login and then did an update with "sudo pkcon update"
While there was only a couple of packages to update it seemed to fix the problem and I could login normally after that.
Hope this helps.

PS: I also needed to manually login to the Wifi network at the prompt before doing the update.
sudo nmcli dev wifi connect "network-ssid" password "network-password"

PPS: Actually had a second laptop with exactly the same issue.
This time the update was done with "sudo pkcon update" but the same login problem happened!
I did a second "sudo pkcon update" and the problem was fixed once again.

Looks like this is an obvious bug with the latest KDE Neon setup!

Ah, ****. Forum ate my first reply. Thanks for the idea... I did manage to get to a prompt but no joy there, neither for user nor root.

One attempt, I only got my username typed in and it suddenly cycled to the GUI login screen.

Also found that the keyboard buffer is not working. Up-arrow and down-arrow produce escape characters, something like >]]A and >]]B.

This looks to me like it's blown off something in the keyboard interface. Maybe that's a clue?

Anyway, I think it's past saving. Time to nuke it and hope for better from the next release.