Use a known-working, MFi-certified Apple cable and wall adapter. Cheap, non-certified cables often damage the internal charging IC.
Often, the phone is powered on but "frozen" with a black screen. A force restart can jumpstart the system. iphone-6-no-power-solution-100-working
Go to product viewer dialog for this item. that won't power on, start with simple software and external hardware resets before moving to advanced board-level repairs. Use a known-working, MFi-certified Apple cable and wall
Use a toothpick or compressed air to remove lint from the charging port. Use a known-working
Press and hold the Home button and the Power (Sleep/Wake) button simultaneously.