Your preview files have somehow become corrupt and you will need to delete the folder that contains the previews. See the screen capture to locate the folder. The highlighted one ending with the .lrdata. Delete the file and then Lightroom will rebuild the previews when you access the folders.
Do not delete any other files in the folder.
Please visit my web site <http://www.hankwilkinson.com/> www.hankwilkinson.com for the latest of my photos
For information on any of these photos please email me or call 612-756-9970