My budgie died suddenly?

she was 2 years old nearly to the day, and she died suddenly, she was fine yesterday, flying round and feeding i have an aivary with 3 other birds in it they are only 8 months old. what could have caused this? and the others seem saddened and are not as chirpy as normal. how long will it take them to get over it and how would i go introducing another new baby in to the cage?