Did you or someone else replace the screen? If the phone hasn’t been stripped down and cleaned with alcohol to displace any water the chances are the board is corroded. The longer it’s left the worse it gets I’m afraid. Totally strip the phone down and remove any corrosion with alcohol and a toothbrush. Look for any solder joints that have been eaten away and component’s that have been damaged. You may need a shop with a microscope to look for you as there are metal plates soldered to the board which might need to come off too.