This is a quote from Tae Kim's Guide
Can someone explain what this form of 見える is? It almost seems like they are trying to make a negative verb into an adverb but is that possible?
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It almost seems like they are trying to make a negative verb into an adverb but is that possible?
Certainly, it's possible. ない conjugates just like an i-adjective. You will see this happen all the time.
I'm not sure it's right to call it an adverb in this case but since it has the same form let's abuse the word. As you know, when you want to describe how something changes you take an adjective in the adverb form and add なる, e.g.
あつい。 It is hot.
あつくなる。 It will become hot.
If you're happy with the above then your example sentence should now be easy to understand:
見える。 It is visible.
見えない。It is not visible.
見えなくなった。 It became not visible. = It disappeared.