Is it a shuttle or round bobbin (rotary)? If its a shuttle bobbin it may be one of the 20x1, or Free BB, or Boye #4, 12 or 14. Alternatitively you might drop a standard 15*1 (Commonly used) about the equiv of the diameter of the shank of the needle. Slide the plates open and turn by hand and see if the tip of the shuttle will enter the loop above the eye of the needle. Making sure the long groove is away from the shuttle. Might need a pice of fabic ulin to help the loop form. If it doesnt pick up the loop, drop it a skosh more til it does.