You need to 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.
N.B. do not touch files ending .lrcat
If your catalog is in a different location it's easy to find when opening Edit / Catalog Settings, change to the General Tab where you will find it's location and can click on Show to directly open it's location.