。
That girl went missing in the fog-shrouded forest.
Literal
That girl [subject-が] fog-hovering forest [possessive-の] inside [at-で] went-away [casual-ちまった].
霧たちこめる is an attributive form used as a modifier — 'fog-hovering' describes 森. なくなっちまった is a very casual contraction: なくなる + ~てしまう → なくなっちゃった → なくなっちまった (regional/older masculine form). Here なくなる means 'to go missing' or 'to die' rather than 'to disappear (of things).' The context suggests 'lost in the fog' or even 'perished.'