Why does my I phone se keep going on and off?

I have a I phone se and my phone said it had a update to download I said yes and everything went normal and it started to downloaded. I went to look back at my phone and I realized it was turning itself on and off over and over again it wont stop its been happening for about 2 hours and I tried holding down the home and power off button but it doesn't do anything. What should I do?