"Lightroom had trouble reading preview caches and needed to close."
In your operating system, go to the folder where your catalog file is located. There will be a sub-folder whose name ends with Previews.LRDATA. Delete this subfolder. Do not delete anything else.
Don't re-import. There's nothing wrong with your catalogue or your images.
In the same directory as your catalogue, there's an item named <catalogue name> Previews.LRDATA. That's the database of all your previews that display in the Library module. If it doesn't exist, LR will recreate it. It's corrupt,. Delete it, and LR should come up normally. It will get rebuilt "on demand" or go to the Library->Previews menu item and rebuild them.
I have 7 lrdat files:
August 22, 2010 ("Lightroom Catalog Peviews")
August 22, 2010 ("Lightroom 5 Catalog Previews")
March 24, 2015 ("Lightroom 5 Catalog Previews")
March 27, 2015 ("Lightroom 5 Catalog Previews")
May 23, 2015 ("Lightroom 5 Catalog Previews")
January 24, 2016 (("Lightroom 5 Catalog Smart Previews")
February 15, 2016 ("Lightroom 5 Catalog Previews")
Should I delete all or just the latest?
No need to delete the Smart Previews one.
You want to delete the one that is in the folder that your Lightroom catalogue lives in. (The catalogue that gives you the error when you try to open it).
Most likely the Feb 15 catalog
Thanks to everyone for their input. Unfortunately, I can't seem to delete the right catalog. I searched the computer for .lrdata files, found the ones I listed, and deleted all of them. But when I opened Lightroom again, I just got the same group of files I had before the project started a month ago. I imported all my photos again this morning, before I raised the issue in this forum, and I can't find a way to un-import them or to get back to where I was yesterday. Generally there seems to be no solution to get back to the condition of the photos and catalog on Feb. 15. Any additional ideas would be very much appreciated.
After you delete the .LRDATA folders, you still have to open the proper catalog file.
Try opening each one by double-clicking on it until you find the one that has the photos you want.
Thanks again. It looks like that may have fixed it.