If all your thumbnails are suddenly showing the same generic picture icon, the problem may likely be a hard-to-find setting that's accidentally been turned off. It's possible that you selected the Adjust for best performance option while solving some other problem. Choosing that option turns off all the special visual effects in the list. Or your computer may be low-powered enough for the default Let Windows choose what's best option to have disabled it.
To fix this, right-click on Computer and select 'Properties'. Click on 'Advanced system settings', then click on the 'Settings' button in the Performance pane. On the Visual Effects tab, be sure the option 'Show thumbnails instead of icons' is checked. If not, check it and click 'OK'.