What I understand is that she had nodules on her vocal cords that required surgery. It left her voice to where she couldn't sing like she did before. This is what I found:

Julie Andrews underwent vocal surgery in 1997 that resulted in permanent damage to her voice, leading to a significant emotional and professional impact.
Background of the Surgery
In 1997, Julie Andrews began experiencing issues with her singing voice, which she first noticed during a Broadway performance. After consulting with a throat specialist, she was advised to undergo surgery to remove what was believed to be non-cancerous nodules or a cyst on her vocal cords. The procedure took place at Mount Sinai Hospital in New York City.