If you do not need to stare at your computer terminal and want to turn off the monitor to save energy or whatever reason then there are a couple of commands at your disposal.
You can use the vbetool utility to turn off the display as follows. First, install the utility using apt.
$ sudo apt-get install vbetool
Run the following command to turn off the display.
$ sudo vbetool dpms off
To regain control of the console by pressing Enter key, use the following command:
$ sudo sh -c 'vbetool dpms off; read ans; vbetool dpms on'
You can use xset to turn of the display using:
$ xset -display :0.0 dpms force off
or turn on the display using:
$ xset -display :0.0 dpms force on
You can set a delay before the monitor goes off to allow all events to be processed by the X server before turning the display off using the following:
$ sleep 1 && xset -display :0.0 dpms force off
Using xrandr to Turn off Single Monitor
You can use xrandr to turn off a specific monitor seeing the other methods here act on all monitors. Run the following command to list your monitors:
$ xrandr -q
Assuming your monitor is identified as CRT1 then you would run the following to turn it off.
$ xrandr --output CRT1 --off
Or the following to turn it on:
$ xrandr --output CRT1 --auto