My guess is that you have set the video recording mode to HD (high definition).

The LCD screen has an aspect ratio of 4:3 and HD video has an aspect ratio of 16:9

To fit the HD video image on the screen, the camera must reduce the size of the image on the LCD.
That will leave a black stripe at the bottom and top of the screen.

You can set the video back to 640 x 480 and it will fill the LCD screen.
If you want HD video, you will have a black stripe at the top and bottom of the LCD screen.
That is normal.

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