whitney houston*(Via ABC News) – Years after the peak of her success, Whitney Houston’s unparalleled voice was “gone” and it took years of coaching sessions to get even part of it back, her one-time vocal coach revealed to “20/20.”

“It appeared to be so damaged that when I first began working with her, I was not sure what I could do,” Gary Catona said.

In the 1980s and 1990s, such ballads as “I Will Always Love You” and her groundbreaking performance of the national anthem firmly established Houston as a singer whose soaring vocals — capable of mammoth crescendos and jaw-dropping chord modulations — had no rivals.

But after years of hard-living, including drug use and smoking, Houston’s voice had become a shell of its former self.

“She’d say, ‘I know I’ve done things that have hurt me … and I have to change it,'” Catona said.

Catona began working with Houston in 2005. By then, he said, “her speaking voice was completely hoarse, and her singing voice was completely hoarse.”

“She had virtually had maybe one or two tones in her lower register, that was it,” he said.

Read/learn MORE at ABC News.

See the full story on “One Moment in Time: The Life of Whitney Houston,” a two-hour “20/20” special, TONIGHT at 9 p.m. ET.

Watch a clip from the show:

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