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. 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. 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. In their 30-minute sessions together, Houston spoke of the challenges of living up to the memory of her own powerful vocals, Catona said. "She used to say to me, 'People have standards that they expect you to live up to. I mean, I can't. ... I have a problem with that, myself, trying to live up to what I once was.'" For all her self-awareness and her dedication to her sessions with Catona, the vocal coach saw troubling signs that the singer was giving in to her vices. "She knew in her heart of hearts that this special gift that she had was being undermined by her own activities," he said. "I left her house with my keyboard, and I had forgotten something, and I walked back in and she had a cigarette in her hand. And she saw me. She said, 'Oh Gary,' and she hugged me, and threw the cigarette over my shoulder as she was hugging me." By 2009, Houston did manage to release what was largely billed as a comeback album, "I Look to You." "When I got her at her best, she was about 75 to 80 percent," said Catona, whose work with her ended that year. The album sold more than 300,000 copies in its first week -- a career record for Houston -- and brought her back to the top of the charts. The reviews were solid, but some were quick to note that there was a clear difference between her latest effort and her performances of yesteryear. "What's hard to give up is the dream of painless perfection that the young Houston represented," a Los Angeles Times critic wrote. "'I Look to You' doesn't soar like the old days, it's fine to hear Houston working on her own recovery plan." The feedback grew more negative as Houston began a world tour and YouTube videos surfaced, showing her voice cracking, seemingly unable to hold the notes she was known for. In 2011, legendary music producer and Houston mentor Clive Davis said Houston would not make another album until her "golden voice" returned.