You have a corrupt preview cache, that's what the message is telling you. To fix, you need to delete the current preview cache (Lightroom will start to rebuild a new one when you then start it). To delete the cache, use Finder/Explorer to go to the folder that contains your catalog. In the same folder will be the file named the same as the catalog but will end in Previews.lrdata, e.g. it will be something like "Lightroom 5 Catalog Previews.lrdata". Delete this file (taking care NOT to delete the file ending with ".lrcat"), then Lightroom should start OK.
Jim Wilde, thank you ever so much, LR now works well. I really appreciate the info. Tom