I signed my baby over to an adoption agency yesterday. Today I realized it was a mistake. The agency said it's too late, the baby belongs to them. Is there anything I can do?

I don't know that there is anything that you can do but talk to a Family Law attorney and do it quickly because if there is anything you can do the statue of limitations will be limited.

That depends on a LOT of things.

You need to contact an attorney ASAP. Like NOW. Tonight.

If you asserted in court that you knew what you were doing, you were not being coerced, you were doing this voluntarily---then your odds are VERY low.

However---this is going to depend on your state. Some states have a provision of a certain time to change your mind. Many states, however, do not.

You really REALLY need an attorney, and you need to get one FAST.

I have to agree... usually, from my knowledge, there is always a time to change your mind. Even after the adoption papers are signed, you still have a certain amount of time to change your mind, depending on the state laws.
As the others said, you need to contact a lawyer immediately, something doesn't sound right when an agency does not allow you to change your mind...