Lightroom error on preview cache or displays gray thumbnails

If restarting Lightroom does not fix the issue, remove the preview folder (Windows) or preview file (Mac OS).

In Windows:

1. Exit out of Lightroom.
2. Navigate to the folder that contains your Lightroom catalog. The default location is in the My Pictures/Lightroom folder under your user folder.
3. Locate and open the [Catalog-name] Previews.lrdata folder. 
4. Drag the previews.db and root-pixels.db files to the trash.
5. Restart Lightroom. If you continue to have the same issue. return to the folder you opened in step 2. 
6. Drag the entire [Catalog-name] Previews.lrdata folder to the trash.
7. Restart Lightroom.

Note: You do not need to delete the [Catalog-name] Smart Preview.lrdata folder, if one exists. This folder contains smart previews, which are different from previews.

In Mac OS:

1. Quit out of Lightroom.
2. Navigate to the folder that contains your Lightroom catalog. The default location is in the Pictures/Lightroom folder under your user folder. 
3. Locate and drag the [Catalog-name] Previews.lrdata file to the trash.
4. Restart Lightroom.

Note: After you delete the previews, your folders might reopen more slowly while previews are being rebuilt.