Hello Nadia,
First of all slacken the stop motion off as if to wind a bobbin and see if the wheel turns when you press on the control, that will rule out a slipped belt problem, next simplist thing to do is force the balance backwards in case there is a small piece of thread jammed in the raceway, it will be very tight, if you can't force it back and the needle is up then take out the bobbin case and the raceway if possible and see if there is thread stuck there, if that is completely free of thread then it will be a process of elimination going through the machine to see where it is jammed.
drivewithken@blueyonder.co.uk, March 2008
