、。
Oddly enough, she suddenly disappeared.
Literal
Strange things [frame-に], she [topic-は] suddenly disappeared.
妙なことに ('strange to say, oddly enough') is a fixed sentence opener — a noun + に adverbial used to frame surprise at what follows. The variant 妙なことには with extra は adds slight emphasis; both are grammatical. いなくなる is literally 'become not-existing' — the predicate いない ('not exist') with ~くなる (inchoative 'become') — Japanese's standard way to say 'disappear, vanish' for animate referents.