It may just be the plastic that has gone, i.e. knob is turning but not turning the spindle underneath. If that's the case contact the manufacturers and get a new knob (they just pull off)
If the spindle is stuck you can usually purchase those from the manufacturers aswell although you'll need to get a gas safe engineer to fit it. Spindles on older gas cookers were easy to repair (just unscrew, clean and re-grease with molecote), newer one's are sometimes more difficult.