Don't panic, I have had this happen also. As others suggested, just clear the bobbin and under the needle plate. I read once that the best way to prevent this from happening is to make sure the thread take up lever is at its highest point before you start stitching. After a while you get into the habit of as soon as you stop stitching, just turning the handwheel a bit until the lever is at its highest point. That way when you start stitching again, everything is good to go!