Please see the attached video, I've got no clue what happened. https://youtu.be/Y7li1KvwnL4
2 Answers
It might be due to the clipping border as Serge mentioned above, you can click on View > Clipping border or use the ALT + B hotkey
Might be your rendering range. I've found how rendering range can ruin an amazing image.
Maybe see this, How do I increase the render distance? might answer your question.