Chunks have dissapeared when I enter my world

I had a world created in version 1.12.2, and I have been playing on it for a long time. When the game got updated to 1.13, strange walls started appearing randomly, on my world occasionally, and when I opened the world for the last time, suddenly many chunks that went all the way to the void dissappeared, including the one were I was, and I fell down and lost all my things. I have a copy of my world, but it is also damaged by this, so I really need a solution.