The miracle Nora hopes for "in terror and hope" is for her husband to change and accept responsibility. She had previously forged a letter to the bank in order to save him from crippling debt. He inevitably discovers the letter. Nora hopes that he will "step up" be a man, and realize that he had ignored their situation, both personally and financially, and assume responsibility. Here is an excerpt from the scene in Act 3:
Nora: When Krogstad's letter was lying out there, never for a moment did I imagine that you could give in to this man's terms. I was so absolutely certain that you would say to him: go on, tell your tale to the whole world. [And when that was done...
Helmer: Yes, what then? When I had exposed my wife to shame and disgrace?]*
Nora: When that was done, I was so utterly sure, that you would step forward and take everything upon yourself, and say: I am the guilty one.
Nora: You think that I would ever have accepted such a sacrifice from you? No! Οf course not. But what would my assurances have been worth against yours? That was the miracle which I was waiting for, in terror and hope. And it was to prevent it, that I wanted to kill myself.
Nora is waiting for Helmer to defend her honor in the face of Torvald's accusations in the letter. Sbe believed that it would have been a miracle if her husband protected her. Instead Helmer reacts violently toward the Nora after reading the letter, setting up the final scene of the play where Nora tells Helmer she does not believe in miracles any more and leaves him, letting the door slam.
Remember that the play is a translation. The use of the word "miracle" is not always considered the most accurate translation of the Norwegian although it is is the most common translation in English. The original phrase meant something more similar to "wonderful thing" something that could happen rather than the "miracle" connotation of something that is unlikely to happen.
The "miracle" that she was waiting for was more hope. She knew that it could never happen. With the tarantella, it was said that if you would have got bitten by a trantula that only wild movements would make the miracle come true. Nora danced the tarantella like her life depended upon it. She wanted the so-called "miracle" to happen.