Why is my phone shutting off and on during an update?

[AC Question]

I have an HTC one m7 and it told me that it had an update. I clicked 'install' and it went to the 'updating software from verizon wireless' screen and the bar started to fill up then the screen went black, the phone vibrated, and the white htc screen came up, then it went back to the update screen and started to fill the bar again. It has repeated this process over and over for about half an hour now. It is plugged in. I don't know what to do or why it is doing this. Please help!