Betty White died of a stroke

The official cause of death listed on the Los Angeles County document obtained by CNN is cerebrovascular accident, the medical term for a stroke, when blood flow to the brain is blocked causing impairment due to lack of oxygen.
White died in her Los Angeles home early on New Year’s Eve at the age of 99.
Best known for her roles on “The Golden Girls” and “The Mary Tyler Moore Show,” White was a fan favorite, adored across generations.
A nationwide movie event will go on as planned on January 17th, to celebrate what would have been White’s 100th birthday. The film, to be shown at 900 theaters nationwide, will follow White’s day-to-day life, highlighting her work and fervent advocacy for animals.
Betty White's funeral will be private, but you can still celebrate her life
White’s funeral will be a private.

Source : Cnn