Hi Abebe. Well, that water damage changes the whole situation. I'm thinking this may not be a software issue but rather a hardware issue that must be address in an authorized service center.

Dear Carl, I disabled the auto brightness, yet the problem persists. But once the mobile got water damaged. 

Hi Abebe. Have you disabled auto brightness? That's causing your phone to only light up when there is sunlight. Pointing a flashlight on the phone should simulate a sunlight to make your screen light up.    

Dear Carl, Thank you for your response. Still the screen light does not function. Unless there is a sunlight. 

Hi. Go to Settings > Display > Brightness, disable auto brightness. Let me know if that's what you're looking for.