Try these options:
1) In the Organizer; under File->Catalog, select your catalog and click Optimize. OR
2) In the Organizer; under File->Catalog, select your catalog and click Repair. OR
3) Try rebuilding the thumbnails as per these steps:
3.1. In Help > System Info, find the folder containing your catalog.
3.2. Exit PSE.
3.3. In that folder, delete the file “thumb.5.cache”.
3.4. Restart PSE.
3.5. Edit > Select All.
3.6. Edit > Update Thumbnail.