Did you by any chance fill the bobbin with nylon thread? I did this once on the Viking 555. The heat generated by winding the thread caused the bobbin to secure itself to the spindle...
I had to break the bobbin to remove it...this was what a SM mechanic told me to do. Just be careful not to break the spindle. It breaks a bit easier if you remove some of the thread....I know, what a waste of thread....but better than a service call.