Comment 33 for bug 1990750

@alextcranglesim
This bug is not necessarily a duplicate of the referred bug
https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-510/+bug/1975650
It just happened to also arise in some cases with Kernel updates but this is just the last instance of it happening.
The black screen with blinking cursor happened to me many times in the past when during a fresh install not installing proprietary drivers and later on installing nVidia drivers downloaded from nVidia.
You can search the internet for 'black screen', 'ubuntu', 'blinking cursor', 'nVidia'... and will find plenty of issues reported but none of the solutions ever worked for me.
The point is that I previously never knew how to handle this situation and resorted to a fresh install as I though my system was definitely broken.
This is until I realized that it is still possible to access the system with some keyboard commands as explained above in order to start up Ubuntu and to restore its proper startup behaviour by subsequently updating the driver to the proprietary tested version, which was by the way only the last step in solving the black screen issue.
The other bug report only provides the code to restore the drivers but not what to do when the system is hanging with the black screen.