When an ending to a story is unclear or uncertain, it's an ambiguous ending.
In these type of stories, the ending can be interpreted in different ways, depending on the viewer or reader's point of view.
An example of ambiguous ending is "The Little Prince" by Antoine de Saint-Exupéry: at the end, the Little Prince disappears, and it is not explained whether he has died because of the snake bite or if he has returned to his planet. It is up to the readers to decide which version they prefer.