MultiMonitorTool is a freeware tool that allows you to easily turn off and turn on a single display monitor from GUI and from command-line, without need of any installation. Be aware that turn off/on a monitor works only if your Hardware supports DDC/CI.

In order to turn off a monitor from the GUI of MultiMonitorTool, follow the instructions below:

• Run MultiMonitorTool.exe, and in the main window select the monitor you want to turn off.
• Right click with the mouse, and then choose 'Turn Off Monitors'
• If you want to turn on a monitor, choose the 'Turn On Monitors' menu item.

  

If you want to turn off or turn on the monitors from command-line, open MultiMonitorTool and look at the 'Name' column or 'Monitor ID' column. The string under these columns can be used to identify the monitor from command-line.
For example, in order to turn off a monitor that its name is '\\.\DISPLAY2', use the following command:
MultiMonitorTool.exe /TurnOff \\.\DISPLAY2

You can also use the monitor number only, without the '\\.\DISPLAY':
MultiMonitorTool.exe /TurnOff 2

If you want to turn on the monitor, simply use the /TurnOn command, for example: (In this example , the Monitor ID is used):
MultiMonitorTool.exe /TurnOn "MONITOR\GSM5B54\{4d36e96e-e325-11ce-bfc1-08002be10318}\0015"

It's also possible to switch between on and off state, for example:
MultiMonitorTool.exe /SwitchOffOn "MONITOR\GSM5B54\{4d36e96e-e325-11ce-bfc1-08002be10318}\0015"

There is also another tool of NirSoft, ControlMyMonitor, that allows you to switch monitor off and on from command-line. The following article explains how to do it:
How to turn off and turn on a single monitor from command line