One way is for a baby that learned to speak a language before "disappearing" and another comes out to learn the present language.

I lived in Thailand for the first four years of my life, and I have no connection to the language even though that's all that was taught and my mother spoke it to me. The one that knows the language was traumatized and dissociated away. Will it come back? I don't know and I doubt it.

I know that I took French in high school, but that's so buried because of the long standing abuse that was going on at the time, I'm sure there's a Frenchie in here somewhere.