What could be the reason for a bald? how could i stop this at the early stage and regrow my lost hair?

Usually genetic: I see you are 22 years old. All normally masculinized men lose the hair on their temples at your age. Many men, especially those blessed with abundant chest hair, go on to lose more in the familiar pattern. Your physician can give you medications, or you can embrace your genetic heritage and masculinity and focus on looking good and being healthy overall. Photo Guy isn't complaining. Cheers.