Stolen information

You can still file your tax return claiming your daughter.  

You will need to mail in your return and attach a statement that the return rejected when you tried to efile it because your daughter's social security number had already been used on another tax return. 

You have sole authority to claim her and attach copies of your documentation (birth certificate and custody documents are golden). 

The second step is to complete Form 14039 Identity Theft Affidavit for your daughter to report the identify theft of her social security number to the IRS. 

This will put into  effect steps to prevent anyone from using your daughter's social security number without authentication that the IRS will provide you to verify it is really you and your daughter using your daughter's social security number for valid tax purposes.

Form 14039 Identity Theft Affidavit