Today, Cubase 13 lost its way to my VSTs

I’ve been using Cubase for around 20 years now and always keep with the latest pro version.
I fired up version 13 today, loaded the project I was working on yesterday and faced pages of errors - telling me that most of my VST instruments didn’t exist!!
I also lost a personalised instrument selection list (on the Rh panel) and it went to the default list.
I have a LOT of VSTs, spread over 4 solid state hard drives - all bought and paid for which still showed up as fine in the licensing software - so they were still there - Cubase just forgot where to find them.
I’m sure the details MUST be held in a file somewhere but, not knowing where to find these, I spent the afternoon inputting all the file paths and rebuilding the personal menu.
My questions are :-
Where are the files with the path and menu information held? Although I backup regularly, without knowing what I’m looking for, it doesn’t help me much.
And
Why did this happen without Cubase showing a single error until I loaded a project?
Surely, the loss (or corruption) of a one or more files should be error trapped somewhere along the way?
I’ve lost yet another half a day, trying to get around the problems of Cubase 13 pro.
I rebuilt the paths and menu by running Cubase 12 and copying the details using VST instrument manager.
Any ideas or info welcome.

Running on AMD Ryzen 9 on Windows 10