Looks like the bobbin case from my Tin Lizzie. Mine did the same thing. It will just pop back in, the prongs will only fit one way. I went and got mine to look at, but I can't see for sure if the wider spaced prongs are at the top or bottom and I don't want to take it apart. ;-) It looks like you have it oriented correctly in the photo, but like I said, it will only fit one way. It happened when I first got my machine. After I popped it back in it never came out again.