Download the factory image for your device here: https://developers.google.com/android/nexus/images
Follow the instructions at the top of the page to flash. Note that the WIFI and Mobile versions have different images. Make sure to use flash-all
otherwise the bootloader and radio will remain the same and the images won't flash.
Another option if you want to keep your ROM would be to download the factory image for the version you have on the device and flash only the boot.img
using fastboot flash boot boot.img
instead of flash-all
. That should restore your boot but you have to know which version you have on the device.