The valve core is easy to locate inside the tube. It has probably settled into the lowest part of the tube. Pinch along the length of the tube until you feel the valve core. You may be able to make it fall back toward the valve stem, but then you will need to locate it again. (Second photo) You will need to move it along part of the way "inch worm"-style. (Third photo) This means you pinch the tube against the end of the valve core more distant from the stem. Then pinch and push some of the tube in front of the other end of the valve and grasp the front end of the valve. Release the pinch at the rear. Pinch where the rear of the valve is now.