Gallery thumbnails appear as gray boxes.

Following my first file import in 2025, my entire Lightroom classic  catalog  suddenly seems corrupted:

In library (gallery) view the thumbnail images are no longer visible, just appearing as  greyed out boxes (albeit with correct filenames when hovering over the box). The images are apparently  still in the catalogue, because if an image is selected and then processed in the develop module, the image can now be seen. However, for the entire catalogue it seems any editing changes (exposure, crop etc etc)  have been lost and only the original raw image is available:  5 or 6 years of edited images effectively lost!

Adobe Lightroom classic 14.1.1

Windows 11 (recent upgrade) 23H2