Why isn't my iPhone turning on?

I received an iPhone 6 to fix from a coworker. I plugged it into the wall, okay so the apple logo appears, then it goes to a black screen which still looks lit up. I unplug it, it turns off immediately. I attempted to plug it into my computer, it doesn’t turn on. I tried holding the home button, power and home button, volume and home button, and power and volume button. None of these seemed to turn it on. I am guessing the battery is the problem, however, I am not too sure. I am not sure if it was water damaged or not. I heard water damage could be a problem. If this is because of water damage, what would I replace? The display works fine. All signs lead to the battery, I just need a second opinion.