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.
open a terminal, and use xrandr command to get identity of displays:
$ xrandr
Screen 0: minimum 320 x 200, current 2304 x 2048 LVDS-1 connected primary 1366x768+417+1280 ... VGA-1 connected 1280x1024+1024+256 ... HDMI-1 connected 1024x1280+0+0 left ...
tell xrandr to revive laptop screen
$ xrandr --output "LVDS-1" --auto
By the way, often laptop screen and external monitors are all blank on resume from suspend, when the system requires a password. It seems that nothing will revive the screen. It is possible to enter the password blindly, and the screens will come on. But you might be able to revive the screens so you can see what you are doing when entering the password, by pressing the "Fn" key.