own release.  She could not live with the despair brought about by the memories, however, and so when she died, the memories returned to the people, who were completely unprepared for them. The Giver himself was overcome with grief at Rosemary's failure, and could not help them with the turmoil that suddenly beset them.

Rosemary had trained for only a short time, and the number of memories she had received was much less than those Jonas already has.  If something were to happen to Jonas, and his memories should return to the people, the results could be devastating (Chapter 18).

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.

Short Answer:  The Receiver who preceded Jonas was released.  

In Chapter 18 Jonas asks the Giver about this Receiver and is told that her name was Rosemary and that the Giver loved her as he does Jonas. He tells Jonas that her training went well until he had to transfer the experience of pain to her, particularly the pain of loneliness and loss. The Giver explains,

"I transferred a memory of a child taken from its parents. That was the first one. She appeared stunned at its end."

After this experience, the Giver gave the Receiver only pleasurable experiences; however, she was markedly altered by her experience of pain. "I could see it in her eyes," he tells Jonas. When, in spite of her agony, she insisted that the Giver continue her education, he imparted to her, not physical pain, but different types of anguish, such as poverty, hunger, and terror. Finally, after the finish of one day, Rosemary kissed the Giver's cheek and went to the Chief Elder, requesting release. "I never saw her again," the Giver adds.

After Rosemary was released, the memories which she held went into the community and they were "overwhelmed" by so many feelings since they had never experienced such depth of feelings before. And, then, the memories were lost forever. Having heard all this, Jonas realizes what a tremendous burden it is that he will bear as he must "lift the burden" for all the others.

The Receiver who came before Jonas made a request for release that was granted.

Ironically, the previous Receiver was the Giver's own daughter, Rosemary, whom he loved; he lost her ten years ago. At first, she was very enthusiastic about her new role, and he gave her weeks of happy memories, memories that made her laugh in delight. But, she knew that she would have to have difficult memories; it was her responsibility. So, although the Giver did not impose physical pain upon her, he did give her loss and loneliness as he transferred the memory of a child who is taken from his parents. "She appeared stunned by this one,"the Giver tells Jonas.

The Giver continues, "I backed off, gave her more little delights. But, everything changed, once she knew about pain. I could see it in her eyes."

Still, she insisted that the Giver not spare her; she said it was her duty, and she was right. Nevertheless, the Giver could not bring himself to give her physical pain. But he gave her many types of anguish, such as poverty, hunger, and loneliness. 
Finally, one afternoon, they finished for the day. The Giver tried to end positively by giving Rosemary something cheerful and happy. "But the times of laughter were gone by then," he tells Jonas. Rosemary stood up, came to the Giver, and kissed his cheek. Later, the Giver learned that she was released.