I saw the "security upgrades" available. Attempted upgrade using
sudo apt-get update
followed by
sudo apt-get -u dist-upgrade
It went through thye usual routine, except it seemed a far larger upgrade than usual, with lots of nvidia stuff. Part way through it asked me to invent a password, which it said would be used to complete the upgrade. I did as it said. It finished the upgrade and went into boot mode, but it will not boot. All I get is a blank screen apart from "Terminate Plymouth Boot Screen".
I can get into the bios, but it does no good. It looks normal except there doesn't seem to be anything to boot!
Help please
(I am typing this on a slow windows machine.)