https://blogs.scientificamerican.com/observations/brain-gain-a-person-can-instantly-blossom-into-a-savant-and-no-one-knows-why/

Savant syndrome comes in different forms. In congenital savant syndrome the extraordinary savant ability surfaces in early childhood. In acquired savant syndrome astonishing new abilities, typically in music, art or mathematics, appear unexpectedly in ordinary persons after a head injury, stroke or other central nervous system (CNS) incident where no such abilities or interests were present pre-incident.

But in sudden savant syndrome an ordinary person with no such prior interest or ability and no precipitating injury or other CNS incident has an unanticipated, spontaneous epiphanylike moment where the rules and intricacies of music, art or mathematics, for example, are experienced and revealed, producing almost instantaneous giftedness and ability in the affected area of skill sets. Because there is no underlying disability such as that which occurs in congenital or acquired savant syndromes, technically sudden savant syndrome would be better termed sudden genius