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Your Lightroom preview files have become corrupt and need to be rebuilt. Go to your Lightroom sub folder in your Picturers / My Pictures folder and delete the file ending with .lrdata. Do not interfere with any other files in the folder he one with the extension .lrcat is the Catalog File.
Then the next time your Catalog Lightroom will generate new previews.
Thank you so much DdeGannes - I am currently doing that now - The pic really helped as I am not the most techy of people! Thanks again! :-)