iPhone 6 will not power up
Iphone 6 will not power up
Black screen, charger plugged in but still black screen
iPhone 6, iOS 12
The support article that you arrived here on should provide you with the steps necessary to try to resolve this issue. If they didn't work for you, your next step would be to make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to have them take a look and provide you with your options.
Ref: Genius Bar Reservation and Apple Support Options - Apple