If you have to leave your Windows 10 PC alone for a period of time and don't want to shut it down, you can turn off its display by locking it, using one of the following three methods.
Video guide on how to lock Windows 10 computer:
Way 1: Lock computer via the user icon in Start Menu.
Open Start Menu, click the user icon and choose Lock in the list.
Way 2: Lock computer through Ctrl+Alt+Del options.
Press Ctrl, Alt and Del at the same time, and then select Lock from the options.
Way 3: Lock computer in Screen Saver Settings.
Step 1: Type lock computer in the search box on taskbar and choose Lock the computer when I leave it alone for a period of time.
Step 2: In the Screen Saver Settings dialog, choose On resume, display logon screen and hit OK.