Hi, I've been struggling to find a solution to a problem that's stopping me from getting past the sddm login screen.
Background: I installed KDE Neon userlts, and then installed the proprietary nVidia drivers for my aging GTX460 gpu; I remember the version was 375. All was well for several weeks and more than one kernel upgrade, until after one upgrade on or around Sept 20, the display appeared to be wavering, and I found that the nVidia driver was not being used, but instead that nouveau drivers were loaded. I then blacklisted the nouveau drivers, and on a whim I installed nvidia-384 from the graphics-driver ppa. Bad move. Xorg and sddm start OK, but when I attempt to log in, the sddm screen freezes. Switching to a tty and back again leaves me with a black screen and an X cursor (which responds to mouse movement).
There are no errors in Xorg.0.log, nor anything obvious in syslog. I tried rolling back the nVidia drivers to 375: same result. I purged all the nVidia 375 packages, removed the modprobe blacklists, and went back to nouveau: same result
Xorg is running, and the nouveau kernel modules are loaded (kernel version is 4.10.0-35-generic), as I can tell by logging in to tty1. However, I get no desktop.
I guess that I could re-install Neon all over again, and I'll do that unless anyone has any bright ideas? Happy to provide info and logs, of course.

OK, another chapter in the saga. Having had no joy with my broken installation, I documented my partition layouts, backed up a few things like /etc, saved a dpkg list, and reinstalled from a bootable USB drive. Just booting from USB proved something, i.e. that my GPU hardware was not at fault. Kwin ran perfectly, I could install widgets and all that. I re-installed the userlts edition of Neon by formatting the / and /boot partitions, while preserving my /home and /usr/local partitions. I also selected to fetch updates in the background during installation.
Install seemed to go perfectly, but on reboot into the fresh install, sddm would still not start Kwin properly. This was a major surprise. The default nouveau drivers were loaded, /etc was fresh, so it had to be something in /home. I dropped into a tty, and searched for all the files that had changed just before everything went pearshaped (ten days ago) with

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find . -daystart -mtime -11 -ls | grep -Eiv "mozilla|chrome|thunderbird" | sort --key=9 | less


from my home directory. The grep clause eliminates a lot of cached stuff for me; if you use this, adjust it for your own needs.
Selecting possible suspects from this listing, I renamed ~.config, ~.kde, ~.local and both kactivitymanagerd and kscreen within ~.local/share.
Running startx from the terminal gave an error message about timeout writing to .Xauthority, so I deleted that, (while X wasn't running!) and I could start an X session successfully with startx from the command prompt, verifying that at least X works.
The current situation on reboot is that sddm will not start KDE at first. If I drop into a tty and do

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sudo systemctl restart sddm

then sddm will start KDE, although plasma crashes sometimes (updates come next...). I restored the renamed directories mentioned above, without breaking anything, so removing .Xauthority seems to have contributed to success. I still have to restart sddm at every boot, though, and I haven't attempted any nVidia drivers, so I'm not calling this as solved, yet.