Use a known-working, MFi-certified Apple cable and wall adapter. Cheap, non-certified cables often damage the internal charging IC.
Press and hold the Home button and the Power (Sleep/Wake) button simultaneously. iphone-6-no-power-solution-100-working
Often, the phone is powered on but "frozen" with a black screen. A force restart can jumpstart the system. Use a known-working, MFi-certified Apple cable and wall
Plug it in for at least 30–60 minutes before attempting another force restart. Use a known-working