Go to your catalog folder and delete the ......Previews.lrdata folder. Make sure you dont delete the .lrcat file as that is the catalog.
Lightroom will then rebuild the previews when open.
Delete all your previews, and the previews.db and rootpixels.db files, and preferably clear the acr cache.
Then select All Photographs, and go to library/previews and choose build all new standard previews. If you have a lot of images, do this overnight.