Nexus 6P Bootloop - Flash "Google" on dark background keeps going on and off forever
It seems like you are facing the unfortunate end for your device. I personally have gone through 3 of these devices that fell victim to the same sudden BLOD (Boot Loop of Death). You have a few options at this point: 


Get yourself on the list to be part of the class action lawsuit against Huawei and Google. 
Contact Google or Huawei (don't have the link for this one sorry...) and demand a replacement. For a while there I know that people were given replacements for this reason well beyond their limited warranty.
If you're feeling particularly savvy you can perform these steps outlined on XDA that will give you functionality back to your phone. I've used this method for all 3 of my 6Ps that got the bootloop. Best of luck, I definitely feel your pain.