iPhone died overnight - at 91%

Before I went to bed I checked my iPhone 6 battery. It was at 91% so I did not charge it. I woke up and checked my phone only to find it dead. I have an iPhone 6 still running iOS 10.3.3 (12/27/17) Please help! (I knew my battery was already bad because it would go from 80% - 68% in about 10 minutes without any use, I had the screen brightness almost all the way down and location services off!)