Welcome!
Is this a recent issue? Did you notice the battery getting hot or draining quickly right before this? I'm assuming that the battery lost all charge. It would be best to hook it up to a charger and let it charge overnight, then try to boot the phone.
Did you try installing any software, like a phone update or a ROM that might have caused the phone to shut off?
Unfortunately, I think we need some more data before a final answer can be given.