Try taking a picture, (I realize that you cannot "see" what you are taking) and then download it onto a PC to see if it is alright?
If the picture is OK then you have a problem with the LCD screen in the camera.
Here is a link to the Ifixit guide on how to replace the LCD screen in your camera.
Remplacement du panneau arrière du Samsung NX300 and NX300M
Here is a link to suppliers of the part. It is not a recommendation to use any of them it is just shown to give to an idea of the price and what it looks like. Just search online for Samsung NX300 LCD screen for more suppliers which may suit you better.
If the picture is not OK, does the menu screen work OK?
If it does then you most probably have a problem with the CMOS sensor in the camera.
Here is a link to the Ifixit guide on how to replace the CMOS sensor in your camera.
Remplacement du capteur CMOS du Samsung NX300.
Unfortunately I have been unable to find a replacement part, perhaps you may have better luck
If it doesn't then you most probably have a motherboard problem. Here is a link to the Ifixit guide on how to replace the motherboard.
Remplacement de la carte mère du Samsung NX300
Again I cannot find a replacement motherboard either.
hopefully this is of some help.
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Thanks so much for taking the time to help me with is. I will now start testing as you suggest. Your instructions are very clear.