Hello Dolla,
You may well have some thread caught in the raceway, the easiest way at the moment if you can't get the raceway out is to force the balance wheel backwards and you may find it will free off, if it does not then you will have to take the raceway out and the chances are you will find the small piece of thread, if you have taken the raceway out already and it is still tight, loosen the stop motion as if to wind a bobbin and see if you can move the balance wheel, if you can't the chances are that the belt has come off and jammed if you have an internal motor, if you can move the balance wheel then it will be a process of ellimination finding out where the machine has jammed.
drivewithken@blueyonder.co.uk, February 2008