Ubuntu and Windows BOTH don't load after grub - but Ubuntu CD will load and run perfectly

This is a weird one. Something has broken, but I don't know what and how to find it.

I left Ubuntu 18.04 running on my desktop and came back to a black screen. Reboot got to Grub, but then Ubuntu boot only starts and stalls at a black screen. Tried Windows 10 and got to the first login screen, but then consistently gives BSOD without stabilising at login screen. Multiple tries gives same response on both Windows and Ubuntu. I infer then that this is not caused by driver errors.

Tried Windows repair, but no luck. Tried Ubuntu disk, which runs a disk based session perfectly. Ran Memtest86 for 20 hours without an error logged, and all other repair protocols. Nothing changed. Not software?

It is a fairly old machine with Intel i7 3770 Quad core 3rd gen processor with 8 G of RAM. Failed primary hard drive replaced a few months ago by new 480G SSD drive with both Windows10 and Ubuntu18.04 rebuilt. Has been running lightning fast since. Not as new is the replacement video card, but it still runs all three monitors perfectly when I get the Ubuntu disk session up.

Any ideas about what may have gone wrong and how I can track down the error? I have another stick of RAM on order to see if changing out the RAM affects things. Both systems breaking at boot suggests hardware rather than software - but the Ubuntu disc will load a session and run perfectly, so it is hard to know what bit of hardware is causing the problem.

Suggestions as to what may be wrong and how I can track it down gratefully received!

Ian