Most likely a tiny piece of thread is stuck under the bobbin area. Happened to me. Sinks Sewing Service in Lebanon, IN (phone on internet) might be able to tell you what to do over the phone. He is a very nice guy who work on old machines and helped me with this once.
Make sure you put the throat plate on properly or it still will not sew correctly. The little bobbin lever needs to fit in the slot under the throat plate. Ask me how I know.