"Sun is the ultimate energy source for all living organisms."
✴This is because plants need sunlight to make food through photosynthesis. This food is consumed by the animals and human-beings. Carnivores indirectly depend on plants for their food. So, if Sun is not there there will be no life on earth.
✴Secondly, sunlight is a good source of Vitamin-D.
Vitamin D is essential for strong bones, because it helps the body use calcium from the diet. Traditionally, vitamin D deficiency has been associated with rickets, a disease in which the bone tissue doesn't properly mineralize, leading to soft bones and skeletal deformities. So if sunlight is not there it can result in loss of Vitamin-D which can cause deficiency diseases.
✴If sunlight is not there there will be no day on Earth. Light will not reach our Earth. Every time there will be only darkness.