You’re not getting the context.
You’ve not considered the question before.
You mis-heard the examiner.
You don’t know a key piece of information.
You’ve gone blank.
You got distracted by a sudden thought or idea.
You’re just not making a connection.
It’s only been two seconds since the question was asked and your brain hasn’t leapt to a response yet.
Going blank or not knowing what to say in the viva is among the top fears of PhD candidates. None of the above are reasons for failure. None of the above are insurmountable, although if they occur in combination they can seem very scary. Nearly all of them are situations that can be improved by asking questions – either of yourself or your examiners. That extra breath, that extra pause, that extra idea, whatever it is, can be useful to shake some ideas loose.
Not knowing something is not the end of the viva. Pause, think, ask questions and work yourself away from “I don’t know…”