Press Windows key + X
Click Device Manager
Expand Display adapters
Right click the current display adapter
Click Properties
Click Driver tab
Click Rollback driver if the option is available
If not
Right click the current display adapter
Click Uninstall
Check the box that says, "Delete the driver software for this device.”
Exit Device Manager
Restart
Go to Start -> Settings -> Update & security, then Check for updates and install any available updates.
Press Windows key + P
Click PC Screen only
Disconnect the external monitor.
Restart.
Connect the second monitor.
Press Windows key + P
Click Extend
See if it starts working again.