If you remove the throat plate, you can get to the back of the knob and its linkages. Heat them up with a hot blow dryer. Use Tri-flow's Superior Lubricant, then keep trying to turn it. Bernina made a special tool that fit on these knobs to help release them. Sometimes it's just a thin layer of schelack-like film that cracks right off. Other times, it's old oil that is thick and like tar. Good Luck!