What if something blocks the sun what will happen

Look up “nuclear winter”. Though the original theory did - as the name indicates - pertain to what might happen if we all actually launched bombs at each other in a sudden escalation of the Cold War, it’s been since applied to climate theory and the effect observed in years where there was a significant increase in volcanic activity as compared to the baseline.

In short, global temperatures would dip, ice would advance, climate would change. In the nuclear winter theory, though, the sun is not completely blocked, its intensity is just diminished. It is assumed that some higher life forms would adapt and survive. If the sun were completely blocked, the effects would be more extreme.

Earth would still have heat-sources - volcanic activity isn’t related to solar radiation in any significant way (not that we know of) - but most plants and many bacteria rely on sunlight for photosynthesis. If the sun were actually blocked out, the entire food chain of most higher organisms (including humans) would be disrupted. Life would not cease to exist - there are plenty of microorganisms and extremophiles that don’t require sunlight - but if the disruption lasted too long, the life that survives on Earth would not greatly resemble the life that exists now. (Most higher organisms rely on sunlight at some level; even deep-sea fish that never see sunlight themselves rely on the dead sinking bodies of plankton and higher-living fish for food. Hope it help!