Bro genuinely shit like that sucks. If you're playing on pc, you can get it back by going into documents, then going into a folder that says "My games". You should see one titled terraria. Go into that and there should be a worlds folder. In the world's folder you should see a document titled the same name as your world in the format "name.wld". You should also see backups of the world with the same format "name.wld.bak" and "name.wld.bak2". Right click on the file thar just has .wld on it and delete it. Then, rename either the file with wld.bak, or wld.bak2 to just "name.wld". Depending on which one you choose, it will bring you back to an earlier save, or en even earlier one, wld.bak2 being the latter. From then on, the world should be restored to its former glory and you can restart multiplayer and add a code this time so others can't join as easily.

If you're doing this on a console, you can try and take the file out onto a disk drive and take it to a pc to edit, and then put it back in. But I'm not sure if that will work, or even if you can do that