Just a heads up, if your Photon/P0 will not power on, check the Heatshield around the P0 module. I had an issue with mine being slightly bend in by a servo and it appeared to create either a short that was caught by the internal power regulator, or it was being reset. The LEDs did not illuminate, and the buttons did nothing. It appeared as if it did not short the external supply, but nothing happened. I bent the shield back and it took off without fail. I bent it back for testing and the same result.
It was bent near the 3D barcode by Pin A3 and A2.
Hope this helps some poor soul who think’s their photon is toast.