I know someone who has recently signed over rights. This person now feels they have made a terrible mistake. Is there anything that can be done?
Thanks

Hire an attorney ASAP, and see if they can petition the court to hear the case. Since signing over the rights has to be done and approved in court by the judge, only the judge can undo it, if it is possible

Was the child adopted by someone else?

One of the reasons a Termination of Parental Rights is rarely granted is because of such remorse. Generally a judge will make absolutely sure that the individual is sure they agree to the TPR before granting it. So the odds of getting it reversed are slim. Your friend needs an attorney to explore the possibilities.