I don't think I met the official criteria of selective mutism as a condition, but I did have quite a few times when I was a child where I just... could... not... speak! It was definitely anxiety-related, but it's probably worth noting that I have ADHD with some subclinical austim traits. (Actually, I suspect if my younger self could be evaluated with the modern criteria, I'd be considered to have ASD, or Asperger's Syndrome in countries using the ICD.) Talking in general has never been a strong point of mine.

Wikipedia says that about 0.8% of people have selective mutism at some point in their lives.