I can find many instances of this problem.
But no support responses.
I have downloaded the 3gb six times now (7th starting).
The 2nd time it happened as per the above description.
I did not wait patiently - I went out and cut the grass and did the shopping.
Black screen on return. I waited 2 or 3 hours and turned the power off physically.
On restart W10 looked like it was starting 100% complete applying updates - it restarted a couple of times.
It then said there was a problem and was restoring my old operating system.
So, that happened twice.
ON THE FIRST TIME - I downloaded.
The information said that I could download and install at a time convenient to me.
I downloaded - and it started immediately.
I tried to stop it and couldn't - I accepted the agreement etc and finally could cancel out.
However, a dialog came up "install now or later?"
I was only given the choice of a time in the next 3 days and that it would automatically happen.
I did not want that!
There was no way to cancel - even with Task Manager - the task purged but popped backup.
So I could do nothing.
This next comment probably does not belong here, but ...
I want to reformat my hard disc and install from scratch - not upgrade.
I cannot copy and run the download files from a dvd for example.
I have to download the 3gb again.
I cannot find any help or information - it's all advertising.
I cannot even find that MANY people have installed successfully and are happy :-(
Even in CNN Technical there is no NEWS.
Here endeth the lesson.
POST SCRIPT
I found out about install from dvd ... yea :-)
Media creation tool --->
https://www.microsoft.com/en-us/software-download/windows10 (i hope)
with instuctions
However, you need a activation key (it says)
unless you have already upgraded the "normal" way :-(
Now I can try the insert install disk and "repair" - wish me (us) luck