You need to go under the location where your Lightroom Catalog is stored and get rid of Previews.lrdata file
By default, Catalog is stored under Pictures\Lightroom
There would be a file with the name previews.lrdata which needs to be deleted or renamed as previews.lrdata-old (do not remove .lrcat file)
In the folder with your catalog (*.LrCat) file, there is a folder similarly named but ending in "previews.LrData".
Exit LR, delete the "previews.LrData" folder and let LR create it from scratch. Do not delete anything else.
Thanks! Do I "backup" LR as I usually do each time I exit?
It is suggested to backup each time if you do extensive amount of editing regularly.
or, you can setup the backup once a day or once a week depending upon the amount of editing you do.