A very small number of iPhone 6s and iPhone 6s Plus devices are affected by a component failure, which causes the device to refuse from powering on. These only include iPhone 6s and iPhone 6s Plus models that are manufactured in a specific timeframe, which is October 2018 to August 2019. This only affects certain serial numbers from these devices manufactured on these dates.
Users can check Apple’s support webpage and enter in their iPhone’s serial number to see if their iPhone 6s is affected by this issue. If it’s affected by this issue, Apple will service the iPhone for free by replacing the faulty component. Service can take place at an Apple Store, an Apple Authorised Service Provider or by mailing to an Apple Repair Center.
Therefore, having an iPhone that can’t turn on is a major hardware issue that requires immediate repair. It renders the iPhone 6s useless as it is a problem regarding the battery or in its power supply. But Apple neither specified what type of component nor any further detail of what is causing the iPhone to not power on.