Apple has initiated a recall for certain iPhones of the 6S series, saying that it has found that the relevant units may become unable to start under certain circumstances. The Cupertino-based OEM will not disclose which aspects of the phones causes these potential failures, but has announced that it will rectify it if those who own these device send them for servicing.
https://www.notebookcheck.net/Some-iPhone-6s-or-6s-Plus-units-may-fail-to-power-on-but-Apple-is-fixing-it.436997.0.html