Just been out taking photos at junior parkrun…?

Why did my screen turn black every time I turned the camera to portrait? Now I’ve transferred the photos to My MacBook and the camera is no longer doing that - the screen stays on unless I put my eye to the viewfinder. Beginner. Be kind. TIA. Also what shutter speed should you use? I went for 500 but some are blurred but it was a grey, misty day so didn’t want it too high 🤔