Why won't it turn on?

It's been about a year since I switched from my iPhone 6S+ to my iPhone XR. I wanted to check to see if my iPhone 6S+ was working and it did not turned on (NOTE: I turned off my iPhone 6S+ so it would not turn on suddenly) it worked when I had to backup but now, it's cold and "dead". I charged my phone, tried to hard reset and whatever I could find on the internet. IT DIDN'T WORK. Help? Or should I just have Apple investigate it because the only damage it got was a crack that slashed the screen in half. But it still worked.