Then trash the preview cache. It’s in the main catalog folder.
Delete the folder with the ending Previews.lrdata
The default location for the Lightroom folder is normally Pictures/My Pictures if you did a standard installation.
Re-launch Lightroom after deleting the Previews folder.
N.B. do not touch files ending .lrcat
Thank you, 99jon, for the fast reply!
I deleted the Previews.lrdata file (you did mean "file" and not "folder", right?) from Pictures/MyPictures. What about the file, "MyMasterCatalog-2 Smart Previews.lrdata"? Should that be deleted as well?
Just delete the folder with the extension Previews.lrdata not the catalog.