I was able to get to the command line using Ctrl+Alt+F1.
Then, I was able to follow one user's instructions on this thread. It was not necessary to recover the system from USB.
/home/ubuntu/ folder there will be a folder
NVIDIA-INSTALLER containing the release. Go to this directory and extract it.
tar xvjf Tegra124_Linux_R19.3.0_armhf.tbz2
Do the following to change the LDK ROOTFS DIR (not sure what that is, but this works ...)
Go to the Linux For Tegra directory and run this script:
Confirm your success (I got all OK's):
sha1sum -c /etc/nv_tegra_release
sudo reboot my GUI loaded after a slightly longer than normal black screen (for suspense obviously).