What will happen if the earth is shrouded in smog with visibility of 500 meters for a long time?

Clouds will cover it, and it will be cloudy. The legendary eternal night. Probably: if the clouds block the sun from shining on the surface for a long time. A large number of plants died without photosynthesis, and animals died without food. The earth has become a big moon. People turn upside down, you come and I go.