Screen blacks out after apple logo

What happens when you connect the phone to an iTunes enabled computer? If the phone is properly booted and powered, then iTunes should recognize it. If it does, then shine a bright light on the screen to see if there is a dim image.

A common failure on the iPhone 6S, especially after a DIY repair, is that the backlight filter gets blown because the fixer forgot to disconnect the battery during the repair. That leaves the backlight circuit fully powered and any misalignment when re-connecting the connector can cause the circuit to blow.

This is repairable but requires a micro-soldering repair and is not DIY.