Hi,
Have you tried turning the power to the Shield off and removing the power cord for 1-2 minutes then reconnecting the power and switching it on to see if it will work then?
If that didn't work and you can somehow access the device then try removing the update file and see if it will download again. It may have been corrupted.
To do so, use any file explorer app (such as ES File Explorer) to delete the OTA folder located in:
Local\NVIDIA\App\OTA
After this folder is deleted, reboot the device, then retry the download and installation of the update.