My iPhone 7 suddenly stopped working and won't turn on

My iPhone7, which I got 4 years ago refurbished, suddenly quit while I was using it. It has a black screen that won't turn on. I tried to Update using my desktop iMac with help from Apple support. But My internet is so slow it said 3 hours to download the software and it quit after some time. I took the iPhone and my desktop to a friend's house with fast internet and tried the same thing. It downloaded the software in 2 minutes. Then it went on to check the software and put it on the iPhone. It then said it was restarting the iPhone, and when it was restarted it would reappear on the sidebar. I waited over 10 minutes, and nothing appeared to be happening. I had the battery replaced about a year ago, and it still said the battery was chargeable to 100%. I believe the battery had been 90% charged shortly before it quit on me, and I followed the instructions to charge it for an hour when it didn't turn on. Any suggestions?

[Re-Titled by Moderator]