I think we can assume that Miranda's mother died before Prospero and Miranda were set adrift at sea by his evil brother, Alonso, and the usurping Duke of Milan, Antonio. Miranda has no memories of her life in Milan (she was only three when they were put into the boat) besides faint images of the women who attended her - no mention of a mother.
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Miranda has not much recollection about her mother. In the story, Miranda came to the Island with Prospero, the former Duke of Milan, who was her father, when she was three years old, which means that her mother should be really old by now.
But, Prospero makes an explicit reference to Miranda's mother in the play, noting that "Thy mother was a piece of virtue"