How was the Portuguese nationality lost — and whose was it?
Portugal — Reacquiring Lost Citizenship
The route back depends entirely on how it went. Two of the historical losses are undone by a simple declaration with backdated effect; one of them has no route back at all. And since 3 October 1981 taking another nationality has not caused any loss, so it is worth being open to the possibility that nothing was lost.
I lost it myself by marrying a foreigner, under the old law → My mother or grandmother lost it that way → I lost it by acquiring another nationality → A parent or grandparent lost it by acquiring another nationality → It was given up by a declaration → It was lost when a former Portuguese territory became independent in 1975 → Nothing was ever lost — it was just never claimed or registered → I'm not sure how or when it happened →