Hey guys. I'll try to describe the short chronology of events that resulted in what I described in the title.
Around month and a half ago, I created a new survival world, and started playing Minecraft after many years of absence. Things were going relatively well, but over the course of my playtime, I got prompted by the game several time to create backups, because apparently I was playing an older version of the game. Which I did. After a while, some bugs started appearing, ranging from small to game breaking. For example, a parrot would crash my game whenever encountered (the only way I fixed that one was to kill it), black boxes would replace the faces of zombies, and Armorer villagers would eventually lock certain trades after a certain number of buys, and would never unlock them again.
All of those bugs were bearable (zombie head bug was fixed after a while), but then one day when I tried to run the game, I noticed that my world had completely reset itself to the beginning. The only thing I did keep was my inventory containing some highly valuable enchanted gear, but everything else - gone. Farms, walls, my house and everything in it. Master level villagers turning back to unemployed and nitwits. All of it. Many hours of work (probably over 100).
First, I tried using the backup which was made few days earlier, but that failed. I also tried using this tutorial step-by-step, but it still failed to undo what happened. Can someone help me out on this one? It's the Java edition of the game. I modded nothing, nor did I play any snapshots (at least not willingly). Just a standard vanilla experience. I also don't understand why did the game tell me that my world was in an older version several times - surely with any new updates it automatically updates the existing worlds too? Any help is appreciated, if I don't sort this out I'll probably uninstall the game.
Well, I copied and pasted the backup world into my .minecraft folder, but the same thing happened - everything was still reset to the beginning. The only difference being is that I was wearing iron equipment instead of the enchanted diamond one.