Hi, best thing I can recommend is first to find the folder of your lost world and back it up (i.e., copy it to another safe location like the Desktop or somewhere where you can keep a "good" copy ("good" being relative since the world doesn't work). At
least you'll have a pristine copy in case your experimentation goes wrong.
Now that you've done that, you can try a few different things. First, I believe the corruption most likely lies in the level.dat and level.dat_old files (perhaps not this last one, but including it just in case). I just ran a little test where I made a
copy of my main world. Then, I quit and created a brand new world and immediately saved and quit. I then copied the brand new world "level.dat" and "level.dat_old" files into the copy of my main world and started that game. It ran and other than the name
of the world changing, it seemed to work. So you might try that.
Confirmed! I had the same problem. After copy-paste the two file .dat from a "new" game folder to the "old" not working, the lost world appeared again and works. The only thing to check/change is (if necessary) the world name.
thanks NeillWhite1 !!