Doctor said "its finished" but grandmother thought that she was going to die that negative thoughts makes her to die

Reason for Grandma's Death
Grandma's death in the story "Portrait of a Lady" by Khushwant Singh is a result of her loneliness and neglect.

Loneliness
Grandma is depicted as a lonely and isolated figure in the story. She lives a secluded life in her mansion, with only her servants for company. Her children and grandchildren rarely visit her, which adds to her sense of loneliness. This profound sense of isolation takes a toll on her emotional well-being and contributes to her declining health.

Neglect
Grandma's family members, especially her son and daughter-in-law, neglect her and fail to provide her with the care and attention she needs. They are more concerned with their own lives and social status, leaving Grandma to fend for herself. This neglect worsens her physical health and accelerates her decline.

Medical Condition
It is implied in the story that Grandma suffers from a serious medical condition, possibly a heart ailment. Her deteriorating health is exacerbated by the lack of proper medical care and attention. Without proper treatment and support, Grandma's condition worsens, eventually leading to her untimely death.

Conclusion
In conclusion, Grandma's death in "Portrait of a Lady" is a result of a combination of loneliness, neglect, and a lack of proper medical care. The story serves as a poignant reminder of the importance of family, compassion, and support in the lives of the elderly.