It's a front loading rotary hook. First, you need to remove the needle plate and see if there's thread wrapped around the hook, and remove it if there is. The needle plate just pops off, no screws. Or, the "finger" that holds the race in place has moved out of position. I'll try to attach a picture. The red arrow points to the "finger," and the blue arrow points to the bracket that holds it in place. I don't have a Pfaff picture, so this is a Bernina, but they are the same concept. You'd loosen the screw on the bracket, put it back in place, which isn't all the way tight. It's more like barely holding it in place. Then re-tighten the screw. It might be something you'd rather have your repair tech do. But here's a picture...