I can't see the thumbnails for prefabs, models, etc.

I recently downloaded Unity 3D, and I’m starting to build my own game, however, I can’t see any of the thumbnail/icons when viewing the models and prefabs. It’s a recurring issue that’s been happening for quite some time, and it prevents me from selecting the correct prefab to use on my games.

Have you tried adjusting the Slider at the bottom of the window?

set the layout to default