Hi @junjunman93 ,
Can you view the menu screens in the LCD display?
If not, try shining a torch at an angle close to the screen to see if you can detect either an image or the menus. It will be very faint so try it in a darkened room to improve the chances of seeing anything if it is there.
''If you can detect an image'' (''or even if you can’t),'' on the screen then there may be a problem either in the LCD display, LCD display flex cable connection to the motherboard or the motherboard.
Try taking a picture and downloading it to a computer. I realize that you cannot see the image in the LCD screen.
If the picture is OK then there may be a problem with the LCD screen, LCD flex cable connection or hopefully not the motherboard.
Here’s a link to a [https://www.youtube.com/watch?v=IDzl2c3AtGI|video] that shows how to replace the LCD screen in the camera. It is also useful so that you can check that the flex cable connections are seated properly on the motherboard.
If required, replacement screens are available online. Just search for ''Nikon D300 LCD screen'' to get results for suppliers of the part.