I then successfully installed the FSDT LAX scenery and I was able to run FS9 and explore KLAX.
This clearly indicates there was nothing wrong in your FSDT KLAX installation.
The following morning after having left FS9 paused at KLAX and minimized to my desktop, I found my computer had crashed and gone into Start-up Repair.
That's not possibly caused by KLAX but, most likely, a bug in one of your drivers and the computer went in Stand-by and probably crashed trying to coming out of it.
I used system restore successfully a little over a year ago after my computer experienced issues with a graphics card driver update.
Exactly, video drivers are usually the worse offenders when the computer crashes when left alone, because of problems with Standby.
Any ideas what has happened?
From your description of the problem, it's clear the problem was caused by System restore.
It has to be connected to the FSDT LAX installation/Cloud9 uninstall.
No, it wasn't. Nothing there can cause a black screen (only low level drivers can) and System Restore is known to be dangerous.