While the best way to deal with a sunburn is to not get one, it does happen.  Sometimes, it even happens when sunscreen is used but not applied or re-applied properly.

Sunburn is your body’s inflammatory response to skin damage caused by too much ultraviolet (UV) radiation. High doses of UV, especially UVB, cause DNA mutations that usually lead to cellular death. The death of skin cells is why people may blister or peel after a sunburn.