Find your catalogue file in Finder or Explorer, and delete the folder called "Lightroom Catalog Previews.lrdata". Do not delete the catalogue file which will be called something like Lightroom Catalog.lrcat
Usually your catalogue will by in your Pictures/Lightroom folder (Mac) or My Pictures/Lightroom (Win).
Restart Lightroom and you should be up and running again.
Thanks so much! That fixed it up
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