My Nokia Lumia 950 seems like it's trying to 'turn over', as when I turn it on it goes to the Microsoft logo, then the blue Windows logo, but shuts off and tries again. It's constant. Also, it's not the battery (I bought a new one).
Is the battery fully charged? What happens when you connect the phone to, say, your PC via USB? If nothing allows the phone to power up properly then I would recommend using the WDRT tool as @ManofGod1000 said.