I am really surprised about this. I knew her from Klitschko.
Panettiere starred in TV shows including Heroes and Nashville along with films Remember the Titans and Raising Helen.
www.bbc.com
"It is with profound sadness that we share the tragic passing of our beloved Hayden," her father, Skip Panettiere, said in a statement to the BBC and other media.
"She was an incredible light and a force of nature who brought immeasurable love and joy to all who knew her and to the millions who watched her onscreen."
The details surrounding her death were not immediately available.
Panettiere played a cheerleader with superpowers in Heroes and had other memorable roles, including in films Remember the Titans and Raising Helen.
Skip Panettiere has asked for privacy "as our family takes time to process this unimaginable loss", in a statement issued through her representative.
Panettiere is survived by her daughter Kaya with her former partner, Ukrainian boxer Wladimir Klitschko.
In 2015, after giving birth to her daughter, Panettiere revealed she had entered a treatment facility to help with postpartum depression.
"It's something a lot of women experience," she told US talk show Live With Kelly And Michael in 2015.
"You think it's, 'I feel negative feelings towards my child, I want to injure or hurt my child.' I've never, ever had those feelings. Some women do.
"It's something that needs to be talked about. Women need to know that they're not alone, and that it does heal."
In her memoir she wrote about her decision to give full custody to her former partner in 2018.
She told podcaster Jay Shetty in May that "the idea that anyone would think that I would just give away my child and be OK with it is heartbreaking and couldn't be further from the truth".