My phones turning on and off, HELP!

My phone was dropped back in June and my whole led screen and battery was messed up. and I got it fixed today, and they replaced everything. then I downloaded IOS 10 and since I downloaded it, my phones been rebooting by itself and about 5 minutes ago, I was looking and testing the newest features and my phone rebooted and kept showing the apple logo on and off for mili-seconds at a time. then my phone displayed the low battery sign. I need help.