Suddenly now, when I open the organizer and click on one of the mutiple folders in the left column, the thumbnails do not show up in the center window. Only a message shows up, saying, "No media to be displayed." I have no idea what has happened. What can I do to get the thumbnails to show up again?
Does it happen for any folder in the left folder panel?
I assume from your question that the files do appear in the browsing space if you are in the normal 'All Media' mode.
Quick test: display your most recent files, choose one thumbnail and look at the properties panel on the right side. Which folder is indicated? Does this folder show your 'No media...' message when selected in the left panel?
Take a small batch of new photos.
Import them in your catalog, but instead of choosing 'from camera or card reader', choose from 'Files and folders' and browse to those photos. Will the photos appear in their folders (the card being still connected to the computer)?
I suspect there may be problem in the catalog about the identification of the drive containing the photos. Since the folder tree is constructed in the left folders panel by the organizer itself, that could be a consequence of reading a 'cloned' disk, which is common today when you replace an original system drive by a cloned SSD drive.