。
After that, the pain never went away.
Literal
She [topic-は] that-after pain [subject-が] disappear thing [topic-は] not-was.
その後 (そのご or そのあと) is a fixed adverbial: 'after that, since then, thereafter.' The construction ~事はなかった (lit. 'it never was the thing that') wraps a clause to assert that the described event never happened. 痛みがなくなる ('the pain disappears') is the ongoing action; ~事はなかった negates its occurrence. Together: 'never was there a time when the pain went away.' The phrasing carries a slightly formal, narrative tone — fitting medical, biographical, or memoir contexts.