Android Nexus5X suddenly died
You should try flashing the factory image for Nexus 5X in Fastboot.


Download Nexus 5X (bullhead) Factory image Android 7.1.1 here or Android 6.0.1 here. And unzip the file.
Setup all necessary drivers
For commands to succeed, you should unlock the bootloader: 
fastboot oem unlock or fastboot flashing unlock. The  device will show  a confirmation screen. (This erases all data on the device.)
Open a terminal and navigate to where you unzipped system image previously downloaded.
Execute the flash-all script. This script installs the necessary bootloader, baseband firmware, and  system.


Hope this helps you out.

If the above don't work, you should try instructions detailed on this thread.