Answer:
In order to answer this question properly one needs to know more specifics about the particular bird. In a young bird, perhaps disease brought home from a pet shop could be a culprit. If the bird is a bit older it could be related to diet. Parakeets fed only seeds develop are susceptible to developing tumors and those often are the cause of death in a bird around the age of two. Perhaps the use of non-stick pots and pans in the kitchen, candles, cleaning agents, sprays could all be suspect. A gas leak of some kind possibly.
Birds seldom die suddenly, in the wild a sick bird will be targeted by predators or by the flock, so birds will hide signs of illness until it is to late.