Some iPhone 6S users find problem with their phone power. They reported that their phone is often turning off by itself randomly and even holding the Sleep/Wake button is not responded by the phone.
This problem is quite serious because it takes quite a long time to turn on the phone again after it shutting down by itself and tapping the Sleep/Wake button not give any help to it. If you get the same problem with your iPhone 6S, you can choose these following options to overcome the problem.
You can try to hold down the Home together with Sleep/Wake button for ten seconds. After you press both of them for several seconds, your iPhone 6S will reboot.
This step might be work but some testimonies states that this is just contemporary resolving your problem. The problem may return not too long after this step brings back your phone to normal for awhile.
Other potential solution to overcome your problem is by reset your phone settings. You can try to reset your iPhone 6S setting by clicking Settings, then tap General, and choose Reset, then Reset All Settings. After this action, you must be aware that you have to re-enter some passwords and settings to your device.
If those steps are not working, you can try to use factory reset option at your iPhone 6S and it is possible that your problem is caused by restoring old data in your phone.
You can do this factory reset but do not restore the backup because it may be cause the same problem again. Please manually set and move the content according to your preference.
Those are some potential solutions that you can choose to overcome your iPhone 6S problem. If those steps are still not working, please contact Apple, retailer or other place that can give any help to your problem immediately.