Try the following-
Find and Rename to hide, or Delete your previews (.LRDATA) folder. It is to be found beside the catalog file and will look something like- " Lightroom 5 Catalog Previews.lrdata"
Do NOT delete your catalog (.LRCAT) file.!
Open Lightroom. It will re-build previews as you select folders or images, so may appear slow for a time.
Thanks a lot, back up and running