From what I can see, it looks like the bobbin winder runs off the hand wheel. So it sounds like if you can turn the hand crank and push the bobbin winder against the balance/hand wheel that it turns. If that is the case, it may be that what is called a stop motion clutch is disengaged.
I'm guessing that the part that I have labeled "A" rotates in the direction of the arrow when you turn the hand crank. If that is the case, then hold on to part "A" to hold it still and turn the knob labeled "B" the opposite direction, and make sure it is tight.
Janey - Neat people never make the exciting discoveries I do.