1An intransitive verb describing an object (e.g. luggage, keys) going missing—its location becomes unknown, independent of anyone's intent.
駅で財布がなくなっていることに気づいた。
I noticed my wallet was missing at the station.
鍵がまたなくなった。心当たりが全くない。
My keys have gone missing again. I have no idea where they could be.
提出したはずの書類がなくなっていて、部内は大騒ぎになった。
The documents that were supposed to have been submitted had gone missing, causing a panic in the department.
2Describes a stock or supply being used up until none remains, or a practice/event that used to happen no longer occurring.
パンがなくなったから、帰りに買ってくるね。
We're out of bread, so I'll pick some up on the way home.
ガソリンがなくなる前に給油しておこう。
Let's fill up on gas before it runs out.
その町の古い祭りは、担い手が減って次第になくなってきた。
As fewer people were left to run it, the old festival in that town gradually stopped happening.
3Describes an intangible thing—pain, anxiety, confidence, a dream—fading away or being lost, as in '自信がなくなる' (one's confidence disappears).
手術後、痛みはだんだんなくなっていった。
After the surgery, the pain gradually went away.
何度も失敗が続き、自信がなくなってしまった。
After repeated failures, I lost all my confidence.