LR obviously does not have previews built for the non-loading pictures.
So I would start to have LR build them: first ensure that the standard size for previews meets your liking in the catalog.file handling settings.
Then select images in Library and use menu Library>Previews>render standard previews.
LR will scan the previews it already has for the selected bunch and create the missing ones.
It may not work for your total catalog in one go, so maybe trigger this for per folder or collection.
If LR cannot render previews for some reason, it will tell you so in a short protocol note at the end. But then you have something technical to investigate from there.
Thank you for the suggestion. As it turns out, the reason the previews would not show up was that the drive the images were on was not online. When I connected that drive, the previews popped up.
What still puzzles me, however, is why, despite my having many of my older images on unconnected, archived external drives, I have not encountered this problem before. I'm used to getting a message such as, 'image is missing or offline', but not a complete blank, leading me to assume that once an image is imported into LR, a preview is made and therefore that preview is in the catalog and will come up (load), no matter whether the image is online or not. Is this not so? Is there something about previews that I'm missing here?
The question is: where are your previews stored?
Normally, without applying some hack, they are next to the catalog.
On my windows7-system that is by default in
C:\Users\[your username]\Pictures\Lightroom, a folder named "[your LR4-Catalog name] Previews.lrdata".
So the previews should always be online.
It may be that LR4 was not complete in building the previews when you closed it last time ?
Otherwise I share your puzzlement. So I would render all standard previews for your entire catalog and then expect that it should not happen again...
Becaues that is the beauty of a complete set of previews: I can do all sort of LR-work apart from Develop while the images themselves are offline - something I would never want to miss!
Yes, the previews appear to be in that folder, which contains subfolders named 0-9 and A-F, all created on 3/3/2011. Each of these contains a long list of other folders, created on various dates in 2010 and 2011. Without looking into each of them, I don't offhand see how these translate into previews of images I've shot in recent months. Any idea how LR accesses these files to load the previews?
Your analysis may be correct--perhaps LR was interrupted as it was building previews for the images that would not load. I'll follow your advice on how to render previews.
Thanks for your help!