One of my Minecraft worlds suddenly disappeared and I tried copying the whole file and going to my saves in Finder to replace it to see if it worked, it would always just be gone when I opened Minecraft again. the file was still in the saves, but I can't see it when I open mMinecraft. I tried using an older version of Minecraft to see if that worked, but it didn't. I tried to put in the items into a new world in my saves folder one by one to check what was making the world disappear when opened, but that didn't work at all. I don't know what to do, me and my friend worked super hard on that map, we had a ton of stuff in it , I'd just really hate it if the world was just gone. Can anyone help?
This is almost certainly the cause of the world being corrupted. That means that something damaged the data of the world, making it unproccessable by the game.
In case you didn't manually manipulate some data in it, it should be recoverable. (A Plugin could also make it unrecoverable, but it shouldn't if it was coded properly)
There is a guide to recover corrupted worlds on the Minecraft Wiki. It is an in-depth and easy to understand guide that should make it easy to recover it.
However, if that doesn't work, I fear you can't repair it as it is impossible to pinpoint the corruption. As a little suggestion: You should make back-ups of your maps regulary.
It should be noted that it can't be fully restored. Your inventory and ender-chest is likely to be empty. Mod meta-data will also be lost (saved information about mods). You will also start at a random location, meaning you will have to find back to where you were and might even spawn underground, causing you to suffocate.
Have a nice day ~Crowley