Brilliant! Uninstalling the Nvidia driver worked. Then I was able to disable secure boot. Then I reinstalled the Nvidia driver.
Here’s what I did:
Uninstalling NVIDIA driver
sudo apt-get remove --purge '^nvidia-.*'
sudo apt-get install ubuntu-desktop
sudo apt-get --purge remove "*cublas*" "cuda*"
sudo apt-get --purge remove "*nvidia*"
sudo add-apt-repository --remove ppa:graphics-drivers/ppa
sudo rm /etc/X11/xorg.conf
sudo apt autoremove
sudo reboot
Disable secure boot.
Reinstalling the driver
sudo ubuntu-drivers devices
sudo ubuntu-drivers autoinstall
sudo reboot
$ nvidia-smi
Fri Oct 4 13:53:43 2019
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 430.26 Driver Version: 430.26 CUDA Version: 10.2 |
|-------------------------------+----------------------+----------------------+
| GPU Name Persistence-M| Bus-Id Disp.A | Volatile Uncorr. ECC |
| Fan Temp Perf Pwr:Usage/Cap| Memory-Usage | GPU-Util Compute M. |
|===============================+======================+======================|
| 0 GeForce GTX 106... Off | 00000000:01:00.0 Off | N/A |
| N/A 59C P0 26W / N/A | 713MiB / 6078MiB | 0% Default |
+-------------------------------+----------------------+----------------------+
+-----------------------------------------------------------------------------+
| Processes: GPU Memory |
| GPU PID Type Process name Usage |
|=============================================================================|
| 0 1606 G /usr/lib/xorg/Xorg 98MiB |
| 0 1958 G /usr/bin/gnome-shell 52MiB |
| 0 2585 G /usr/lib/xorg/Xorg 391MiB |
| 0 2797 G /usr/bin/gnome-shell 105MiB |
| 0 3785 G ...quest-channel-token=4785359086748578459 62MiB |
+-----------------------------------------------------------------------------+
Many thanks Generix!