This one was a mystery for a while...
To Prevent this happening to you:
When you put your bobbin case in the hook always align the case with the silver bar horizontal and the wire loop at the top. It is important to push it in on the left side of the case or top & bottom, to prevent the jam.
To Fix a jam:
Remove the hook with bobbin case and the bobbin from the machine.
Push a pencil or screwdriver through the hole in the back of the hook, while pushing on the bar of the bobbin case, this will release the bobbin case. There is a video to show this. Click Here (to follow)
NB This only refers to models with the Bernina Hook; B4, B5 B7 machines.