Prosecutors alleged she killed 2-year-old Daytona Robertson. The child went into a coma in February 2008 after being taken to a hospital with severe injuries. She died several days later.
Cornejo said she loved Daytona and that she was not responsible for the girl's injuries.
Prosecutors said they will retry the case.
In an unrelated case, Cornejo's biological daughter, 13-month-old Kailee Hundley, died a month after Daytona when she strangled on a car seat strap at the home of her day care provider.
(Copyright 2009 by The Associated Press. All Rights Reserved.)