To prevent this from occuring in the future, choose Lightroom > Preferences > Presets (Mac OS) or Edit > Preferences > Presets (Windows). Uncheck Apply auto tone adjustments.
Hope it helps.
"Apply auto tone adjustments" was not checked.
Please refer Thumbnails display black in Lightroom CC 2015.2/6.2
For existing photos, 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.