I can't vouch for if it actually works or not, but apparently there are some "decompilers" that exist. Depending on what engine you used to make the game initially, it might exist.
I came across a decompiler that takes the Exe from GameMaker 8.1 files and can apparently decompile that Exe back into a GM8.1 file or something like that.
No idea if they work, no idea if you should trust them (I have not used anything like that), I would run it in a sandbox environment or a hardened VM if you really wanted to try something like that.