Kelemvor is right. Shocking the pool is the right thing to do. Did your chlorine level drop too low? Continue to stay at shock level until you can go from sundown one evening til within 2 hours of sunrise the next day without losing more than 1ppm of cl and also have no more than 0.5ppm of CC. At that point, you can let the cl drift back down and keep it back in the correct range per the Best Guess Chlorine chart.