Soft Bricked Nexus 6P
I don't have a full answer for you Ican only help with the issue of your device not being recognized. But since I can't comment I will answer instead.
For your problem with your PC not recognizing your device. When your device is in fastboot mode, and connected to your PC. Goto device manager, there should be an item that matches your device that has a yellow mark showing it needs attention. Update the driver for that item and select the adb driver this should make your PC recognize your device.
This assumes you have the adb drivers installed on tour PC.