Solved (kind of):
I had to go into Mediabay and manually re-scan each library by right-clicking each library folder (under VST Sound). The small tick/check box icon then changed from grey to white.
This made the Samples appear again in the MediaRack (right panel of main window), which is the main way I access them - but this didn’t make the artwork appear when clicking on Loops and samples???
Having had a couple of re-boots and leaving overnight - I looked again this morning and all the library artwork had re-appeared?!
All my libraries are the standard ones that come with Cubase (Analog Techno, Blockbuster etc etc) - I’ve never had this problem before on C11 or since moving onto C12 and C12.02 update?
Anyway - all sorted now - but weird that it happened in the first place?