I’m genuinely not sure what would happen because it seems you would reach a stalemate.
If the defendant refused to get back on the stand, they get told off and held in contempt of court, which results in them being sent back to jail. But they can’t just stay in jail forever with the trial still halfway through, so I don’t know what else would be available to the judge in terms of a stick to obtain compliance from a defendant.
I suppose you could make her go back on the stand and then just let the prosecution finish asking its questions, she refuses to answer at all, so the prosecution just carries on through its questions in much the same way as a police interview carries on through the same set of questions, even though the detainee says “no comment”.
But then I don’t think they physically manhandle defendants who refuse to attend court into the court room, so I assume they wouldn’t do it for a defendant who refuses to return to the witness box.
Maybe they have to just accept that she won’t finish giving evidence, and the judge makes some sort of comment to the jury to the effect that they can decide to take her refusal to return to the witness box into consideration, and as evidence against her when they deliberate?