Hello Margaret,
Are you sure it is bobbin trouble, check everything else first, threaded up correctlt making sure the thread has gone through the tension discs and not to one side, make sure the presser foot is up when you thread the machine, top tension between 4 and 6, bobbin tension enough to pull the thread through with some resistence and the bobbin turning anti clockwise as you look at it, if it still does not sew after that you will probably need to check the timing.
Ken
drivewithken@blueyonder.co.uk
drivewithken@blueyonder.co.uk, January 2008