Depending on your computer/video-card setup, you might need to manually enable the second display port in your Windows Display Properties
From Windows Support Site:
Open Screen Resolution by clicking the Start button , clicking Control Panel, and then, under `Appearance and Personalization**, clicking Adjust screen resolution.
Click the drop-down list next to Multiple displays, click Extend these displays, and then click OK.
The Multiple displays option appears only when Windows detects more than one monitor connected to your PC.
If you don't see an option named Multiple displays in your display
settings, then Windows isn't detecting a second monitor plugged into
your desktop computer. Or, it is not detecting an external monitor
plugged into your laptop. When Windows detects this monitor, it will
appear in the drop-down list next to Display. If you don't see it
listed there, try clicking Detect.
Make sure the monitor you want to extend your desktop to is turned on
and plugged into the correct video port on your computer. You might
also need to select the correct source labeled on your monitor's
controls to ensure the monitor is using the correct video port. For
more information, check the manual for your monitor or go to the