This doesn't happen very often - black screen problems are rarely fixed. Most often they are repaired with either System Restore or a reinstall of the operating system.
We got lucky today in several respects.
The customer knew what he was trying to do just before the black screen occurred (trying to install a USB->VGA adapter).
He also knew when it happened (last night).
The first clue was an entry in the Startups for flvga_tray64
Not much info, but it was likely a startup program for a (yep, you guessed it!) USB=>VGA adapter
Searching for similar drivers didn't yield much - and trying to download and extract files from the download wasn't possible.
So then I searched the startups for "Last Accessed" timestamps.
I found around 6 of them - and they were either directly related to the adapter, or they were related to a video settings program.
So, I disabled them all.
And, viola! I was into Windows!!!
I found the USB->VGA adapter in Control Panel...Programs and Features
And I found another program related to porting video out that was installed back in March - so I uninstalled it also.
Did some updating, cleanup, and housekeeping with the OS.
Then tried to install the USB->VGA adapter - and it worked like a charm!
But I credit the success in this to the owner - who was aware enough to remember what he was doing and when he was doing it.
After that, all I had to do was hunt for clues! Gosh, I love a happy ending! :0)