Using Windows Explorer or Mac Finder, go to the folder containing your Lightroom catalog - usually inside the Pictures or My Pictures Folder under your username.
Delete the folder whose name ends with Previews.lrdata. Do not delete anything else.
Now launch Lightroom, and the previews will be rebuilt as you browse in Library. You will not lose anything.
The cache is the previews you see in LR. Find where your Catalog is located on the hard drive, and delete ONLY the file with the extension .lrdata. not the .lrcat file.
Anything you were working on should not be affected by this, the previews will be recreated as and when needed by LR.
It is obviously important to back up your photos, don't forget the backup that LR reminds you about is only the Catalog and not the actual photos themselves! These should be backed up on a regular basis in can of a hard drive failure or other reasons which there ar many.
Thank you so very much!! So that worked for all but one catalog. I have restarted it after deleting the previews, and it still comes up. So weird. So I am all backed up, just have one session that I hadn't been able to get to backup before this happened. Any other suggestions for the one that doesn't want to follow the rules?! Thanks!
Could it be that you are getting two catalogs mixed up, and that you are deleting the preview cache for a different catalog?
Having multiple catalogs is in most cases unnecessary, and is not recommended, it makes your work more difficult, and can create confusion. To delete all preview caches on your computer, close Lightroom, and do a search for Previews.lrdata folders, then delete them all.
Most people only need one catalog, and to merge all catalogs into one, follow this procedure:
Open the catalog that you want to be your "master" catalog.
Then go to File > Import from another catalog, and repeat for all the catalogs that you have.
In the end you will have only one catalog, which is probably all you need.