Delete the preview cache.
It is a subfolder whose name ends with Previews.LRDATA. You can find this in the same folder as your Lightroom catalog.
Delete only the subfolder whose name ends with Previews.LRDATA, do not delete anything else.
Thanks for replying.... I tried finding that folder, but with out any success, could you be a little bit more specific ?? Thank you
It's probably in your Pictures folder, then Lightroom. It will be a folder within that Lightroom folder.
You are a genius, thank you