What should I do if both yellow and green LEDs light up on the charger?

Both yellow and green LEDs on indicates a deeply discharged or defective battery pack. Allow the battery pack to remain in the charger for 15 to 30 minutes to see if it recovers to normal voltage range, at which point the red LED should come on. If the red LED does not come on after 30 minutes, the battery pack is defective and should be replaced. (Page 9)