Apple has launched a new repair program for owners of the iPhone 6S and iPhone 6S Plus handsets that have had an issue with the power on their handsets.
Some models of the handset have issues trning on, this is apparently due to a faulty component, you can see what Apple has to say below.
Apple has determined that certain iPhone 6s and iPhone 6s Plus devices may not power on due to a component that may fail. This issue only affects devices within a limited serial number range that were manufactured between October 2018 to August 2019.
If you believe you have experienced this issue, please use the serial number checker below to see if your iPhone 6s or iPhone 6s Plus is eligible for repair, free of charge.
If you have had this problem on your iPhone 6S or 6S Plus rthe you need to contact Apple at the link below to arrange a repair.
Source Apple