I did this one time...I have the #555...
You may need to break the bobbin to remove it....that is what I finally had todo after talking to a repair man. I used a pair of needle nose pliers and just grasped the green edge and gently twisted. But first try to use the pliers to remove the bobbin.
The cost of a bobbin is much cheaper than a repair.
PS...No more cheap thread...quality machine...quality thread