I saw the meaning of the words individually, but when I've tried to join them it doesn't make sense.
何者 = who; what kind of person
- 何者: "who", "what kind of person"
- で: "to be", the te-form of the the copula だ
- も: (Used between an interrogative and a negative expression, and adds a meaning of "(not) any" or "(not) at all". For example 何もない means "there is not anything.")
- なく: "not", the continuative form of ない
- なった: "resulted in", the past tense of なる meaning "to result in", "to became"
So the literal translation is "I (or she/you/etc) resulted in not being any kind of person". More naturally, "I ended up being nobody" or "I became nobody." While なくなる can mean "to be missing", this meaning is not relevant.
Context is missing, but this kind of phrase is usually used when someone was once famous/popular/important but became forgotten/isolated/retired.
何者 can also be translated as "nobody".
No one can compete.
No one [Nobody] was in the room at the time.
なにもの【何者】 : あらゆる人。いかなる人。何人 (なにびと) 。
Sicne「何者でもなくなった」is the past tense of「何者でもない」
And 「何者でもない」 means "I'm nobody"
「何者でもなくなった」should have the meaning "I was nobody"