I upgraded a notebook with Windows 7 to Windows 10. It was working fine. I did a Windows Update and noticed that it installed an updated NVIDIA graphics driver. After a restart, Windows would boot only to a black screen. The mouse cursor would be visible and could be moved, but nothing else was displayed on the black screen.
Solution that did not work: Many online solutions advise to press Space and enter password, just as if logging in blind. This did nothing on my notebook.
Solution that worked: The solution provided by Microsoft here worked for me with some changes:
Download the Windows 10 ISO from here.
Create a bootable USB stick from this ISO. I used WinUSB as described here to create this in Ubuntu.
Boot the computer using USB stick. Go to Repair your computer -> Troubleshoot -> Advanced Options. I did not get the Restart option given in the above guide. Instead I chose to restore from an earlier recovery point, which thankfully was present.
Windows 10 was restored back to the time before the update.
To prevent such problems in future, I disabled Windows from updating my hardware drivers as described here.