When you hit F12 at power up to get the boot choice menu you may see the usb listed twice, once for the legacy mode and again as a UEFI boot. It's important to choose the right one. See this thread...
Before I go out and buy a new disk. Just one more go at fixing this one.

The above quote interests me but I'm not clear on it.

I am not using a usb I'm using a Disk I burned from a MS ISO of Win10 install.

You are saying, I think, that if I have the install media as USB stuck in the computer then F12 on boot will throw me to the BIOS and show me what you say and give me a chance to choose the right one. Right? I read the link, it seems pretty certain I'm right.

So I'll have another go and see. Try F12 with the disk in and see what the BIOS Boot menu offers me and if there's nothing I'll Rufus a USB stick.

Thanks for the tip. That's a very clear link.

I've got this underlying major question: how could such a thing happen? What has happened to my pc? This just came out of nowhere and on a machine I've suspected of having a bad board...

:)

I don't suppose this can work with a DVD Disk because the computer doesn't read the Disk to know what's on it, right? So I need to create a USB install?