The IRS is claiming that my dependent is invalid and took $3,000 away from my return. Nothing has changed since she was born and nobody else can claim her as a dependent.

@lcampbell1203 TurboTax does not have access to your tax information.  Once you file your tax return the IRS does not share information with TurboTax for Privacy reasons.,  IRS does not notify us about the status of your tax return.  You will need to contact the IRS to provide information to show that you are entitled to claim your child.  It is not all lost.  You must follow-up with the IRS.  Use the contact information on the letter you received from the IRS.

Someone has possibly used her on their taxes. Contact the IRS and they'll simply ask for documentation that she's a legal dependant of yours that was in your household for at least 6 months in 2021.