Since posting this question I have deactivated the NVIDIA driver that was recommended and now the computer boots without an error message.
In the course of all this, I tried all three NVIDIA drivers none of which worked. However the recommended driver had been working fine until the last couple of days. The problem seems to have coincided with the latest updates - I downloaded them either early this week or late last week - one of which had something to do with X11 (I think) -- NVIDIA graphics driver suddenly stopped working https://bugs.launchpad.net/bugs/322673 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
