This most likely happened because you opened the world in an older version, hence the "unknown update" warning, as the game only recognizes worlds last loaded in itself or versions older than itself (this message can also occur on worlds from before 1.9, when save versioning was added; always check with the latest version to be sure that it isn't from a newer version though), and you should absolutely never downgrade a world, which can even result in total destruction of everything in loaded chunks (the game will completely regenerate them from scratch and only your inventory/player data will survive, possibly not even that if you downgrade far enough).
Otherwise, the game has a serious world corruption issue which causes chunks to randomly move around, which can even happen for no obvious reason at all (you play like usual then the next time you open the world it is corrupted):
MC-161823 Chunks can occasionally be misplaced upon loading a world
In any case, this is why you make regular backups of any data that is important to you, otherwise, it may as well not even exist - I can't even imagine losing my first world, which will be 8 years old in a few months, or all of the mods I've worked on for almost as long (thousands and thousands of hours of work in total):
Schofield's Second Law of Computing states that data doesn't really exist unless you have at least two copies of it.
If you only have one copy of something, it's a bit like Schrdinger's cat. Is it alive or dead? You have to look in the box to find out. In the same way, you'd have to load your single copy of a file or database to see if it was "live" data or corrupted (dead) or had simply disappeared. (Perhaps you deleted it by mistake or forgot where you stored it.)
Ideally, you should have at least three copies of everything, preferably on different media. It is a good idea to store one copy in the cloud, as then you have data "off premise" -- buildings have been known to flood or burn down -- as long as it's not your only copy. Having three copies means you can do file comparisons and therefore check if one of them has been corrupted.
-schofields-three-laws-of-computing-and-avoid-disasters/
If you are lucky you can try right-clicking on the folder for the world and select "restore previous versions" from the popup menu (at least on Windows) and there will be a recent backup that was automatically made by the OS, but do not rely on this.