I play in creative, and I basically had a house with things connected to it with a path. I used the same wood for the other rooms but every little bit of my house is gone! Even my 6-7 dogs are gone and their dog house!
Minecraft stores it's level data in 16x16 block chunks. Very very rarely, your computer (or whatever) will have data corruption issues in the storage of one of these chunks (more likely if the chunk is very full of chests or mobs or modded items), and when this happens, it can result in everything that you built in the corrupted chunk being deleted and replaced with newly generated blocks. If the version you are playing with has had no changes to world generation, it will be like you never built anything there.
I have had this happen twice in five years of play.
The only thing that can be done is to build again, and keep a backup world after you finish with a build.