http://www.askvg.com/fix-windows-explorer-doesnt-show-thumbnail-previews-in-windows-vista-and-7/
It might be due to a bug or the user accidentally turned off thumbnails using Folder Options or System Properties.
Anyway if you are also facing this problem, you can use following easy fix to show thumbnails again in Windows Explorer:
1. Open Windows Explorer, click on Organize button present in command bar and select "Folder and search options".
2. Now go to View tab and make sure "Always show icons, never thumbnails" option is not enabled i.e. its disabled.