tModLoader stores worlds at "Documents/My Games/Terraria/ModLoader/Worlds". They are split into two files actually. The one with the "wld" extension is pretty much just a vanilla world save file, the modded data is stored in a "twld" file with the same name. You can then open the wld file in TEdit just like a vanilla save. (There are probably guides on how to use TEdit somewhere.)
With that said, usually modded worlds get "corrupted" when tModLoader messes something up with the modded data, so this might not help much. When the vanilla portion of the save messes up you usually just get a big empty file, which is unfortunately not very recoverable.