There are an estimated 1,314,967 posts about this already in the forum. Consider doing a forum search before posting yet another of the same, please.
A plain ol' internet search turns up a veritable sh'load of them too:
But to answer your question:
Delete your "catalog Previews.lrdata" folder (delete the whole folder, and only that folder). It's in the same directory as your catalog (.lrcat file)..