My sister's Nexus 5X won't turn on. It is charged. When you try to power on, it makes it to the Google animation but freezes and goes back to black. You can make it to the debug menu go through the options, but when you choose Start it does the same thing. We've tried the Google Support suggestions, and nothing is working.
Nexus devices are pretty simple... Try a factory reset in recovery, and if that fails reflash the factory firmware image from Google. If that doesn't work chances are good the device is defective. The processes are well documented on-line... Normally I would document the process here, but the Nexus/Pixel devices are a special case since there is so much support for them online.
BTW, the condition you are in is called a "bootloop", might want to investigate Nexus 5x bootloop online a bit... The N5x has some history with it.