Using your system browse go to the folder that contains your catalog. Delete the folder that has the extension .lrdata. This is the folder that contains your previews. After you have done that you can start Lightroom, and it will create a new preview folder and will begin to rebuild the cache.
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That error message indicates that your preview cache files have become corrupt. This is not a critical problem to solve. Just delete the previews file it carries the same name as your catalog file with the extension ending "Previews.LRDATA" see the screen capture. Delete this file and when you next restart Lightroom it will rebuild the previews.
The link below will let you know where the file is located.
thank you to both of you... it really helped... thank you...