Phone won't turn on / charge / hard reset

So a few days ago, my phone froze in an app (snapchat). Pressing the home button or power buttons didn’t do anything so I held the power button to restart. Now it won’t turn on / charge / reset. No combination of button presses elicits any response from the phone. It had maybe 70% battery when it last shut off. My question is, how can I diagnose what the problem is? I think its a motherboard issue, but if it could be the battery, I am comfortable replacing that. Any thoughts?