Galaxy S4 won't boot up. Won't turn on

Tried all known troubleshooting and nothing works.

- During night I Noticed that the display came on and off twice so I just shut down the phone. Next morning it would not come back on.

Battery was fully charge when I shut it down. Even hooked it up into PC with USB and nothing. Computer can't find it because it's not coming on. Left plugged into PC and it feels warm so it's on charge but that's not the issue.

I was told perhaps it's the motherboard- just wonder if anyone had any idea of what else it could be. It's only 2 years old and flawless physically, buttons are good, battery is good just can't think of what could have happened.