Bottom half of my world is grey when playing on Nintendo Switch

My son's minecraft world seems to be corrupted. In his creative world that he plays on NINTENDO switch (its a cartridge,  not purchased online) the bottom half of his world is missing.  So you can only see the tops of his buildings, not the bottom or the ground. Please help, he is so upset and spent 7 months building his world. He said he saved it every time he closed the game. Is there ANY way to retrieve it?? This happened yesterday.