It is possible it was disabled in error, following these instructions will show you how thay can be turned off or on. Thumbnail Previews - Enable or Disable
If you want to speed up browsing around in explorer, you might think about disabling thumbnail previews in folders. Note that this works in Windows 7 or Vista
To make this change, click the Organize button on any folder, and choose Folder and Search Options from the menu.
Click the View tab, and then check the Always show icons, never thumbnails checkbox.
Click OK, and you’re done.