Some background, from when all was working:
I was attempting to get better video performance by loading a Nvidia driver for a (very old) graphics card. The driver installation was not going to completion (in terminal) so I got the idea that KDE should be stopped, and then restarted, after the driver change. I was working in terminal with service lightdm stop, and then start, and when that didn't do anything (or so I thought) with sudo /etc/init.d/kdm stop, and then, start. That didn't do anything for the driver installation, either.

When I attempted to reboot, I was locked into a login screen loop, preventing me from accessing the Plasma Desktop.

I was able to get back to the LinuxMint KDE Desktop by running in tty1, from the login screen, and then, in terminal:

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sudo startx

However after noting Mint seemed to be working, I did a logout, upon which KDE crashed, with the following error messages on a black screen:

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Xinit: unable to connect to Xserver Resource temporarily unavailable
Waiting for Xserver to shut down (1)
server terminated successfully (0)
closing log file
xinit: server error

and this is at the top of xsession errors log file:

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/usr/bin/xauth:  /home/ron/.Xauthority not writable, changes will be ignored
Xsession: X session started for ron at Sat Feb 11 17:45:08 WET 2017
No protocol specified
xrdb: Resource temporarily unavailable
xrdb: Can't open display ':0'
No protocol specified
xhost:  unable to open display ":0"
No protocol specified
xhost:  unable to open display ":0"
No protocol specified
Error: unable to open display :0
OpenGL version not found.
No protocol specified
No protocol specified
$DISPLAY is not set or cannot connect to the X server.

To summarize, startx gets the desktop running again, but when I try to exit, or logout, I get a crash.

I am running the system restore utility Systemback, and performed a complete system restore from terminal, but to my surprize, the login-screen loop remained. Apparently the the damage I created involves something out of the 'reach' of Systemback restorations?

May I provide more information, or does the about suggest a problem that can be remedied without just reinstalling Mint?

This is with Mint 8.0 and KDE 5.28.0

Apparently it's resolved.

Mint 18 KDE just went through its first successful reboot, in days, without hanging at the login screen.

With credit to a post from 2009, I deleted the .Xauthority file.

This is only the first reboot that went to completion, so if it starts failing again, tomorrow morning, I'll come back and reopen the help request.