Windows 10 may not see your second monitor. That is very annoying of course, but fortunately there are various solutions.
In general, there are a number of general steps that you can take when Windows 10 does not recognize hardware. That also applies to your second monitor. Think of restarting the system, checking the settings on the monitor itself, checking whether the cable is correct (or try another cable) and checking whether the monitor works on another PC or laptop. There are a number of obvious things, but what if none of that works?
Troubleshoot Windows 10 monitor issues via Settings
You may be able to resolve the issue through Settings. Go to Settings / System / Display. At the top it says Change the layout of your screens. At the bottom right of the picture is the button To detect. Press and then follow the steps on the screen. Have you gone through the steps? Then everything should now work and you can continue to set up the monitor.
Fix Windows 10 monitor issues via Device Manager
If the above doesn't help, you can dig a little deeper and possibly fix the issues through Device Manager. Open for that Start and type the word Device Manager in. Open the option from the menu and in the new window double click Display adapters. Right-click on the adapter you want to update (sometimes you have one, sometimes you have two: update everything).
Select the option Update drivers and then Automatically search for updated driver software. If Windows 10 now finds an update, it will be downloaded and installed automatically. Once this process is over, you will need to restart your computer.
Reinstalling a driver (driver) may also help. Follow the steps above until you right-click the driver for your display adapter. Now choose the option Remove device.
A pop-up will appear where you need to check the Uninstall the driver software for this device. Confirm your choice, restart your computer and go to Device Manager again.
Now right click on the name of your computer and select the option Search for changed devices. Now follow the steps on the screen so that Windows 10 can reinstall the correct drivers for you.
If the computer fails, no problem. If you right-click on a display driver again and under Properties for Driver / Rollback Options If you click on it, you will again get your old driver back.