What would you do if you went blank or froze or could only think “I don’t know”?
It’s a situation you wouldn’t want; it could even be stressful. It’s reasonable to think about it before the viva but unhelpful to worry about it – particularly because there’s a lot you could do in that situation.
You could pause, take a moment to think and then respond. Your first response might be “I don’t know” but perhaps another moment of thinking will help you find more to say.
You could briefly reflect on why you don’t know something. Different reasons prompt different actions. Perhaps you can check your thesis. Perhaps you can ask a question. Perhaps, after reflection, you can just say “I don’t know”.
You could take a moment to think: even if you don’t know, you can share something with your examiners that will demonstrate your knowledge or skills in an appropriate way.
It’s good to reflect on this possible situation before the viva. It could be stressful, but there’s a lot you could do – if it happens at all.