Her father confirms her death to TMZ ... telling us her death was unexpected and a total shock to the family. We're told there were no known health issues prior to her passing, and her cause of death is not currently known.

Mandisa -- who competed on season 5 of 'Idol,' finishing in 9th place -- was found dead in her home in Nashville on Thursday.

She had previously opened up about her mental health struggles, telling ABC News in 2017 that she had contemplated suicide before. However, it's not clear whether her past battle with depression is related to her death.

Mandisa found success as a Gospel singer in the years after 'Idol,' winning a Grammy Award in 2014 for her album, "Overcomer."

Christian radio station K-Love praised Mandisa for her incredible talent and big heart in the hours following her death.

David Pierce, the station's Chief Media Officer, added ... "Her kindness was epic, her smile electric, her voice massive, but it was no match for the size of her heart. Mandisa struggled, and she was vulnerable enough to share that with us, which helped us talk about our own struggles."

He poignantly added, "Mandisa’s struggles are over. She is with the God she sang about now. While we are saddened, Mandisa is home. We’re praying for Mandisa’s family and friends and ask you to join us."

Mandisa was 47.