my iphone 5s keeps randomly restarting

HI, so my iphone 5s just started randomly restarting. the screen wouod turn blue, then turn off, and restart. I also updated my phone to ios 9.1 last night. I had my phone taken away sunce august and i just got it back yesterday but this restarting thing just started this morning and i dont know whats wrong. someone olease help it would be greatly appreciated. thanks!