Most likely you have a corrupt Monitor profile.
Please refer How do I change my monitor profile to check whether it’s corrupted? - The Lightroom Queen and change to Monitor to srgb and let us know is that helps.
Sorry to say that does not work. I use and Eizo ColorEdge CG243W monitor and calibrate with a ColorMunki Photo device, always before the software gives me notice to do it. Also tried changing to several other profiles, as Akash suggested, with no success.
Are there question mark icons or exclamation point icons in the top right corner of the rectangle surrounding the photo?
Nope, DJ. Not one of the images, in any of hundreds of folders, shows its preview. But it is reading the directory, for sure. For instance, I can select any image (without being able to see the preview) and use a contextual menu to "Edit in Photoshop" and have it open normally. Well, almost normally. I get a dialog box that tells me Photoshop's location has changed, and asks to update it. But the point is that the file opens from LR, even though the previews are not available.
Ok, here's something really weird. I've been in the Library module the entire time I've been trying to get images to be visible. Just for the heck of it, I tried the Develop module, and low and behold, previews came up when I clicked on an image. But here's the weird part - when I kept clicking on successive images, only 3, or at the most 4 images would remain visible at any one time. When I'd click on a 5th or 6th image, the previews of those first clicked on evaporated. I could keep making more images visible, while those clicked on earlier disappeared. What's more, those opened previews were still visible in other modules - Library, Slideshow, Print - but I was unable to click on an image in those other modules and have them come open there. Only in the Develop module could I open previes. What the heck?
My guess is that you have a corrupted preview cache.
In Develop previews are created on the fly, whereas Library relies on the preview cache.
Then, using Windows Explorer / Mac Finder, go to the folder containing your catalog.
Delete the folder named [your catalog name] Previews.lrdata
Do not delete anything else.
(if you have any missing originals, and the LR previews are all you have, rename or move the folder instead of deleting it, in case you want to extract the previews later)
Now launch Lightroom, and the previews will be rebuilt as you browse through the Library.
If you want to rebuild all the previews at once, click on All Photographs in the Catalog panel, select all thumbnails, then go to Library > Previews > Build Standard sized previews.
This will take a long time for 100,000 images, so do it overnight.
Thanks, Per. I'm betting you are right. I'm in the process of optimizing right now, which has been going on for several hours. If that doesn't work, and I think it's doubtful that it will, I'll do as you suggest. Thanks again for your help.
In the end, it wasn't necessary to throw away all the previews. As I said above, I optimized the entire catalog, which may have been the sole reason all the images became visible again. Maybe it had something to do with the fact that I had to force quit at the end of that process. Or maybe a combination of the two. For whatever reason, I can see the images again, though the previews look a little softer than before. I chose to Render Standard-Sized Previews, anyway, and all should be good when that is done. Thanks to all of you for your help.