Thumbnails of videos or photos is an excellent and efficient way to visually locate the content you are hunting for. Usually Windows does a fine job at maintaining the thumbnail cache; generating thumbnails in real-time isn’t conducive to a fluid user experience after all.

Unfortunately, like any cache, the contents can become corrupt for no apparent reason.

The simple solution is to have Windows clean-up or otherwise delete the Thumbnails cache. Search and run the Disk Cleanup utility from the Start->Search box.

In the files to delete box locate and check the box next to Thumbnails. If you wish to keep the other file types be sure to uncheck any box next to them before clicking OK to purge the Thumbnails cache.

Returning to any folder in Windows Explore that has the thumbnail view selected will have Windows rebuild the cache into a working state.