PC screen goes off when starting Fortnite?

As you have already read, my monitor suddenly goes off when starting Fortnite and I have already looked at the cables and everything is okay there, my monitor is on but there's a black screen where it is out of range.

Can you help me?

The refresh rate or resolution may have been changed in the graphics or video settings, which the monitor probably can't support / display; therefore he switches off. What can help is to reset the settings to default.

Or try the key combination Alt + Enter after the screen went black - with this you can switch between full screen and window display and you can possibly get back into the game.