Three years before Storm Front, she was shot. According to official knowledge, it happened in a crossfire between John Marcone's men and a Jamaican gang, after which she lived for three weeks in intensive care and died after her life support was ended.
In reality, Amanda had been accidentally shot by Marco Vargassi intending to shoot John Marcone. The Vargassis then covered it up, having the comatose girl officially declared dead, while moving her to a different hospital, provided for by a dummy corporation's trust fund. This was to avoid the unfortunate consequences of the shooting: while her continued existence was of no matter if they were never found out, she could be produced alive if they were ever tried for her murder.
Amanda continued to live in a comatose state, but not on life support, in a private hospital in Wisconsin, a long-term-care and therapeutic facility, known only as a Jane Doe.
In the series
In Storm Front, Karrin Murphy tells Harry Dresden that Greg Beckitt and Helen Beckitt's daughter was shot in a crossfire between John Marcone's men and a Jamaican gang. Allegedly, she lived for three weeks in intensive care and died after her life support was ended.
In Death Masks, Marcone acquired the Shroud of Turin to cure her. On his way to the hospital, he was followed by Harry Dresden; the latter told him he could keep the Shroud for three days, but afterwards had to send it anonymously to Father Forthill at Saint Mary of the Angels in Chicago, which Marcone did. Dresden guesses she is in her late teens or early twenties.