Okay, I entered the boot menu and ran a diagnostic, just to get the computer to do something. It re-booted afterwards, and the Windows 10 menu reappeared saying it was running "Automatic Repair"....

Okay, so I reserved my free copy of Windows 10 a few days ago, and tried to install this morning. It all seemed to be going smoothly... then everything disappeared, and I was left with a black screen...