On the base plate of the sewing machine, under the arm, you should see a large silver button. This is the safety clutch pin.
You will need to press it down completely and hold it while turning the hand wheel towards you. Like other industrial sewing machines, you will probably have to press down slightly on the machine foot in order to turn the wheel.
Turn the wheel both towards you and away from you until you feel the pin engage and press in a little further than it had been before. Once you have the pin fully engaged, start turning the hand wheel towards you.
You will hear a few small "pop" noises as you turn the wheel (again try turnign it both towards you and away from you), but eventually you will hear one very loud and quite distinct "POP" which means you have successfully re-engaged the safety clutch. Once you have done this, you will see the bobbin spin while turning the hand wheel.