Yesterday, when working on my Minecraft clone, BuildWorld, I met a fatal error, which resulted in the failure of any trees to generate at all. I tried to fix it, but the probelm spread, and now the terrain itself refuses to generate. I have the .exe version of the game, which works perfectly fine. Can anyone help me decomplie the .exe, so I can use that, and go back to when it did not have a bug?
No. Learning to make backups is what we all do. Decompiling will result in a bigger mess imo. Decompiling doesn’t generate it like it was before you compiled it. It generates a mess which you have to clean up further.
Just chalk it up as a loss (it sounds like a good learning experience) 
What was the fatal error? Maybe there is a way to salvage your game.
If you happen to be using Windows Vista/7 then there might be a chance to get a older version of those files. There is a backup feature in Windows that makes backups automatically so you might be able to restore the files to a older version or simply test the files from the past and see which 1 works.
I couldn’t find a proper tutorial, but I belive that it was as easy as Right clicking on the HDD on MyComputer > Properties and then look around… should not be too hard to find and it’s easy to use. It saved me once, but I have a feeling that this feature might be turned off by default… hope this helps.