how to restart Win10 install after hung up after 2h30m?

My installation proceeded uneventfully for 2h30m, to the end of the 'preparation' phase. But then, the screen simply turned black and then grey on an endless 2 second cycle. After an hour of this I forced a complete shutdown. But
when I attempted a reboot, the result was simply the resumption of the flashing! Thus rendering my machine unable even to revert to Win 8.1.

I had to use an older computer to access this site searching for help. Good: I found what looked like a perfect post here, written on 30 July 2015 by KLEMENHE (in another thread). He noted this exact situation, and said it was mostly occurring on notebooks, which also were running an external monitor. Yes! That was me! But his thoughtful fix simply did not work.

But remarkably, 24 hours later it rebooted fine, into the original Win 8.1, and all seems to work. Very strange.

So my question ought to be simple: after disconnecting the external monitor, I just want to either resume the install (the big new installation files in C: are presumably intact, if I can figure out which are which?), or start entirely from scratch.
But so far I have found no link via Microsoft to do that. Help!

Since you are good in Windows 8.1, this is what I would do:
Important precautions to take before upgrading to Windows 10 - Windows 10 Forums

You can substitute an external hard drive for the SD card in my tablet upgrade scenario above.

You can get Windows 10 ISO files to either make a USB flash drive or DVD from or mount the ISO file and run setup.exe from the mounted ISO file to start an upgrade.

Windows 10

OR

Tech Bench

Also note, when you start the upgrade to Windows 10, you don't want anything connected to the computer that does not have to be. That means disconnect extra monitors, extra drives, anything USB other than a keyboard and/or mouse if you need them.

This is something you can try, download the Windows 10 ISO from TechBench. Right click the ISO and choose Mount from the menu. Open File Explorer, you will see a new drive. Double click on the new drive to open it. Double click on Setup.exe to start the upgrade.

Windows 10 ISO download

If the above fails, you can upgrade to Windows 10 with a clean install. With a clean install you will lose all your personal files and programs, so back up everything you want to keep. I would also recommend to make a backup image of the entire hard drive before you try the upgrade. That way if something goes wrong, you can restore the Windows 8.1 image. Don't forget to make the rescue USB or DVD. I use Macrium Reflect Free.

Guide to Upgrade to Windows 10 with a clean install.

Download link for Macrium Reflect Free

@ NavyLCDR
I have to try and type faster. lol

Dear Navy LCDR, Dear Trust Me:
Thank you both very much for your remarkably rapid responses! They seem very helpful indeed. I have to go out now, but wanted to thank you quickly. I will try everything tomorrow.

To Navy LCDR, TrustMe: I had further delays, but did work thru your much appreciated suggestions. There were some ambiguities in one of the replies, but by doing a little cherry picking from both, I eventually managed to successfully install Win 10. Gasp!
Thank you both once again.

After this I wanted to also convert my previous machine, with Win 7, to Win 10. I was nervous about that, as the machine's origins are (thru no fault of my own) a messy mix of its original Win Vista and a DVD that was not Win 7, but an upgrade from Vista to Win 7. Which caused grievous trouble several years later, despite Microsoft's official help. So I went to good old SevenForums for help. And there you were, NavyLCDR!
I tried to follow your link on that site, but could not get past the ISO stage.
(I am posting this text here rather than on the SevenForums site, because of what followed below)
So I went the official Microsoft site route. And to my surprise, that went through entirely smoothly. Victory! Thus once again proving my guiding philosophy in life: YOU NEVER KNOW.

-koalady.