Second monitor not working

If your second monitor is blank or not detected, check the monitor power first and confirm the cable is firmly connected at both ends. 

If you are using a docking station disconnect the power and reconnect it

Use the Windows display shortcut to detect displays and make sure the screen mode is set to Extend rather than Second screen only or PC screen only. 

If you are using HDMI or DisplayPort adapters, unplug and reconnect them, because loose adapters are a common cause. 

Restarting the laptop while the monitor is connected often forces Windows to detect the screen correctly.