In Grid mode in the Library module, all your preview thumbnails are black in Lightroom CC 2015.2/6.2.
To correct this issue, install the Lightroom CC 2015.3/6.3 update. Launch Lightroom and choose Help > Updates.
This issue occurs when Auto Tone has been applied to your images.
For existing photos that appear black, select all your images in Grid view, and then click the Reset All button in the Quick Develop panel. Then, click the Auto Tone button to properly apply this feature.