The IRS rejected my daughter's return because they said she was claimed as a dependent on another return. I did not claim her. Does this mean her identity was stolen?

[Opus 17]

Possibly.  Is there another parent, grandparent, or relative who might have claimed the child?  If you live together unmarried with the other parent, did that person list the child on their return, even if they did not want to claim the child as a dependent?