Why all of a sudden some models became full blacked out? Whilst rendering.
Show us a screenshot of your model set to monochrome.
It’s because you do not have any environmental light in your scene or it’s turned off.
Just go to the Settings and turn on environmental light and you’ll be good to go.
Alternatively, you can also add a domelight and it will reflect the HDRI you’ve added to your domelight.