iPhone 6 Stuck in Apple Logo

For whatever reason, for the past week my iphone 6 (that I recently bought online used but in excellent condition) kept on shutting on and off. And then all I could do is turn on the phone but when I got to the apple logo, the screen turns black but it was still on. And now I can't even turn on the phone unless I plug into the wall charger but I still get the same apple logo then black screen, but when I take out the charger, the phone automatically shuts off. I left the charger in for a few hours just in case the phone was out of battery but it doesn't seem to be the case. So I tried to connect the phone to iTunes via usb to try to restore it but the phone doesn't turn on or anything like it should when I charged it to a wall. I thought it was maybe the charger but I connected it to another iPhone which connected to iTunes perfectly fine so this isn't the case either. So I was wondering what's going on and if I can fix it in anyway? Or should I return the iPhone and either get a replacement or just buy a new one?