Part of my minecarft build disappears when i open my world, Not just that one! ALL OF THEM! HELP!!!!!!!

Can you please help me? Before I opened it it was just fine, When i rejoined my world a part of my build is gone! and normal world generation generates around it! It doesn't happen in that world but it happens in ALL on them! Please fix ASAP!!!! If you could help me get my lost blocks back that would be great!