It could be a small stuck thread but it's more likely that you didn't get the bobbin case back in correctly. Once you take off the feed dog plate, you must put the bobbin case back in with the little "finger" in the little notch or the case just swings around and will not stitch. There is a good picture in the manual. If you don't have a manual, they are available on line. Many Featherweights have been sold as "broken" because of this problem.