My hero 3 will not turn on ,

It won't turn on, will not charge, I've replaced the battery, removed the memory card, plugged it in with and without the battery with and without the card, no red led no display , checked the power cord with my gopro piggy back battery. It will charge. And won't turn on with the back battery. I did notice the camera was getting warm last time I charged it, but it still worked, it sat for a couple weeks unused then would not turn on.Can you help?