Hello Hannah,
First of all loosen the stop motion as if to wind a bobbin and see if the balance wheel turns, if it does then the chances are you have a pice of thread stuck in the raceway area, you need to take out the bobbin case and the raceway and get the thread out, if you take the bobbin case and the raceway out you should be able to turn the machine, if you can't then you have a problem elsewhere..
Ken
drivewithken@blueyonder.co.uk, January 2009