Laptop screen remained blank after resumed from suspend, though external monitors were working.
Could not do anything about it because 'Displays' dialogue opened on the primary screen, which was the laptop screen. Don't want to simply reboot, because can't easily check what needs saving, and indeed may not be able to save if can't see primary screen.
So how get the laptop screen back to life?
This was, by the way, after I had entered the user password. It wasn't one of those common occasions where the screens are blank when you are trying to enter the password.