Many pop-up tub stoppers use an arm (inside the mechanism behind the lever handle that you turn) that pushes down on a spring that forces down on one end of a see-saw lever to lift the stopper. When you turn the lever up the spring is pulled up to release the stopper lever. Sometimes the nut that holds the spring falls off under the seesaw lever and the arm is not attached to do the job. Remove the two screws that hold the top plate and remove the arm assembly to see if it is in one piece. If not – the repair becomes more obvious.