If the Sun disappeared suddenly, we wouldn't know about it for about 8 minutes ... that's how long it takes for heat and light from the Sun to get here to Earth.
8 minutes after the Sun disappeared, the sky would turn totally black. All plants would immediately stop growing ... including all the plants that people or animals eat to live.
Then, after a few more minutes, we would start to notice that it was beginning to slowly get cooler, and that would continue until Earth was too cold for anything to live on it.
