My phone was charging lastnight while I was sleeping, green light is o, can't turn on. help!

I believe my phone is a Galaxy 6 - I bought it 2 years ago. My phone was charging last night while I was sleeping. I woke up the green light was on indicating it is charged and now it won't go to my login screen. It is black. no buttons work. I can not power it off nor push the button at the bottom of the phone to return to the home screen to log in. Not sure what is going on. I downloaded an update about 2 days ago. Could that have something to do with this? If not, how do I get my phone back?

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