Starvation or heart attack (parakeet)?

ok we went out of town for 3 days and left our parakeet at home with my dad.when we came back it seemed fine it was making birdie noises as usual. the next night after we came back it was sitting on the cage floor.it was really fat and not moving but it was alive.there was no food in the container.right away we filled it up.there was some water still left but we filled it up anyway.the next morning we found it dead in the cage :( we went on the computer and was talking to our cousin who has lots of birds and experience with them.we told him what happened and he saidi it was cuz of a heart attack cuz one of his birds died like that.my dad was supposed to take care of it so maybe he forgot to feed it. what happened?