Jeff has an older HP Pavilion laptop. He bought a new copy of Windows 10 Pro and installed it, but he's had an issue with the video display extending to his second monitor. It won't work. Leo says that he has a hunch fixes like this will be addressed with the service pack 1 update next month. Leo has always said that you shouldn't upgrade to Windows 10 unless you know your computer is compatible, and Microsoft won't offer it until that compatibility is in place. The fix will be coming, and it may already be there under his computer manufacturer's support site. But it's also possible that it may not be compatible at all. Leo advises being patient, and waiting until Microsoft says the computer is ready.
The chatroom also advises pressing WindowsKey + P, and selecting the PC screen extend option. There's a technote on how to do it at tech-recipes.com.