Going blank in the viva isn’t a big problem. The potential for it might feel worrying, but typically it doesn’t happen that much. If it does happen there are things you could do. You can take your time, you can think and something will come. Going blank doesn’t mean you’re not talented, it means you’re human.
Your viva isn’t a test of perfection.
If you go blank in the viva, sit with it, pause, relax and try again.
Going blank before the viva is also a small problem. If you realise you don’t know much about your examiners, or you’re unsure about the procedure for the viva, those are small problems. If you try to think about a certain chapter or theory and your brain doesn’t supply ideas, you have a small problem.
Small problems often have many solutions – generally either thinking, reading or asking.
If you go blank before the viva, ask yourself what could make it better, and the answer to that question will probably resolve things.