Unforgettable! Inside Natalie Cole's Heartbreaking Funeral

See photos of the star-studded funeral service for Natalie Cole.

David Foster paid tribute to his friend, who passed away on December 31, 2015.

The Reverend Jesse Jackson attended the "Unforgettable" singer's funeral.

The singer underwent a kidney transplant in 2009, after struggling with problems related to Hepatitis C, a rare lung disease causing shortness of breath, dizziness and sometimes chest pain.

Stevie Wonder paid tribute to his long-time friend.

Courtney B. Vance and Angela Bassett attended the celebration of her life.

She "responded well to pulmonary arterial hypertension-specific agents over many years, during which she performed many concerts world-wide, but eventually succumbed to intractable right heart failure, an outcome that unfortunately commonly occurs in this progressive disorder," the statement about her death said.

The singer's longtime music director, Gail Deadlock, expressed shock at her death: "Nobody knew this was going to happen. We mourn the fact that she's gone. But we are celebrating."

Nat King Cole's daughter had a decades-long career with hits including "Inseparable," "This Will Be," and "Our Love."

Smokey Robinson spoke about his friend at the service.

Cole won six Grammys for "Unforgettable" in 1991. Her star on the Hollywood Walk of Fame was adorned with flowers after her sad passing.