If you had to pause your PhD – for medical reasons, for personal reasons, for pandemic-related reasons – then you can absolutely explain that to your examiners. I think it should be enough to say, “Oh, it was disrupted for personal reasons,” and you’re done. A PhD is important, the viva is important, but the work that goes into them shouldn’t be put on such a pedestal that day-to-day human life is overshadowed by them so completely. But you can say more if you want to.
Most of the time, when someone asks me about anything to do with the viva, my first thought is to direct them to think, “Why?” But absence, whatever the reason, was probably quite personal. I don’t think your examiners need to know “Why?” – so perhaps think “How?” instead.
Think concretely and clearly about the impact that the absence had. Did it pause things? There was a gap and then you had to start again. Did you have to change your plans? Explore the differences brought about by the delay. Did you have to leave things out? List what didn’t make it into your thesis.
For absences, reflect on how it had an impact over why it happened.