Did you have Root? Custom Recovery? Custom Kernel? Xposed Framework?
Never run an upgrade with any of these is place... If that was your issue you will need to reflash things by hand.
I assume if you had one of those conditions that your bootloader is unlocked? Yes? And you have your PC set up for ADB (Android Debug Bridge)? Yes? 'Cause you need both of these to proceed.
Download the latest Factory image (KTU84P) and extract it.
https://developers.google.com/android/nexus/images Open the folder and extract the image-<model name>-<version#>.zip. Open the image folder...
You will need the following from here: boot.img, recovery.img, system.img.
Power down your Nexus and wait a couple minutes. Hold the Volume Down + Power buttons until you get to the bootloader screen. The one with the robot lying down.
Plug your Nexus into the USB port of your PC. Open a command prompt.
You should get a return if all is connected properly.
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Code:
fastboot flash boot <file path>\boot.img
Wait for it to finish.
Code:
fastboot flash recovery <file path>\recovery.img
Wait for it to finish.
Code:
fastboot flash system <file path>\system.img
Wait for it to finish.
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You may not loose any data. No guarantees though.
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If Root / Custom Recovery / Custom Kernel / Xposed Framework was not the issue this might still save you... But I would suspect that one of those is the cause of your dilemma.