So, this happens to me all the time also - from the month I got the phone (Nov/Dec 2015) in fact.<p>It seems to die with battery as high as 60%, though it only happens in cold weather conditions, and then immediately switch back on if plugged into a power source.<p>It's definitely cold-dependent in my case, and only started to show same problems again in November of 2016.
<a href="https://getsupport.apple.com" rel="nofollow">https://getsupport.apple.com</a><p>make an appointment. they're replacing a lot of batteries these days. also, check with your credit card (if you bought it with one) as many cards extend the warranty by an additional 12 months. That might come in handy.
I heard it's an issue with the battery. My wife recently took her phone in and they replaced her battery because it is a known issue. Might want to take it to the store.