What happened when the last receiver of memories failed in The Giver?
When the last Receiver of Memories failed, the memories that she already possessed were released and lost. "It was chaos," the Giver comments.
"They really suffered for a while. Finally it subsided as the memories were assimilated. But it certainly made them aware of how they need a Receiver to contain all that pain. And knowledge." (Ch.13)
The Giver's explanation causes Jonas to reflect upon his new position in which he will be forbidden to reveal his knowledge unless called upon by the Elders. So, there would be a major part of his life that he must keep secret. Also, he learns from the Giver that rarely does the Committee of Elders call upon the Giver for advice, although he would really like to advise them, and even suggest their doing some things. But, the Giver observes, "[L]ife here is so orderly, so predictable—so painless. It's what they've chosen." (Ch.13)
Further, when the Giver mitigates what Jonas's science instructors have taught him about the brain, saying, "They know nothing," and insisting that the most important thing in life is memories, Jonas is shaken.
Indeed, this is a pivotal moment in the development of Jonas's character as the Giver's tale of the previous Receiver and his remark about the importance of memories, plants a new seed of thought in Jonas which then furthers the narrative, as well.
Unfortunately, some of the memories given to her by the Giver were released into the community, and members of the community were ill-equipped and unable to deal with having to experience such things as pain and anguish. She was released, or, sent away from the community. It was ten years until the Council of Elders felt comfortable selecting a new receiver of memories... Jonas.
This ties into the greater theme discussed by Jonas and the Giver, over whether or not people should have choice. Jonas has a difficult issue with people around him who do not have the ability to see beyond not being able to see things for what they are. They can't even tell the difference between a blue or red tunic in the morning, and this upsets him. He tries to get some of the others to experience what he does, such as Asher or his sister, Lily, but he is unsuccessful.
The Council was very careful about choosing a new receiver, as they did not want any other memories to be lost forever, or, to be spilled out into the community again. It would be chaotic and disastorous.