I went to Settings>Privacy. It says Screen Lock Off
How can that be? If I close the laptop and then open it again, I find a screen showing the date/time in big characters. Then I press the Esc key and now the system prompts me to enter a password (by the way it does not say if it is the root password or the regular user password; I tried with both but it rejects both!). And it shows a button with 'Unlock' written on it! So it has locked the screen despite the fact that Settings>Privacy says Screen Lock Off.
EDIT: This is Ubuntu 18.04.