Don't use WD-40 on your machine. Now you'll have to clean any areas you put it on. It will gum up eventually. You should get as much of it out of there as you can with a rag and get some Triflow oil or Kroil. You may have to let it sit for a few days. If that doesn't work on releasing the bobbin case there are other products that deal with rust specifically, that would be my next option.