After the first trip from The End City and coming back to the Overworld the whole terrain around me disappeared. I couldn't move and interact with anything. I tried teleporting somewhere but it didn't work, I only saw one chunk of terrain. After relaunching the game nothing changed. It's my most important survival world, if there's a way to save it I would be very thankful.
I'm using the Optifine version btw
Thank you Loek Van Vucht for an advice! Launching the game on vanilia version fixed it.
What happens when launching the game using vanilla?