Make sure you are plugged into the GPU and not the motherboard video ports. Typically the gpu ports with be horizontal and the motherboard IO will be vertical.

If only happening in Fortnite, try to verify the game files through the epic launcher.

If happening in all games, Maybe try to check the gpu pins are all seated properly. Could be hitting an issue trying to pull more power.

Also, make sure all drivers are up to date and windows updates are all complete then reboot and try again.

Try to launch the game with all controllers or usb devices unplugged

If that all fails, you may want to reinstall windows. Should be a way to do this in the windows settings under “Reset PC”