Apple has admitted there’s a component that goes faulty in some iPhone 6S and 6S Plus handsets that can keep them from powering on. It’s now offering to replace that component free of charge with a new repair program.
Apple’s support page doesn’t offer any details about which component causes the issue, but you can enter your phone’s serial number there to see if it’s eligible.
The program covers affected iPhone 6s and iPhone 6s Plus devices for 2 years after the first retail sale of the unit.
Further information here – https://support.apple.com/iphone-6s-6s-plus-no-power-issues-program