。
She was very worried about it.
Literal
She [topic-は] very was-minding.
気にする ('to mind, worry about') frames concern as 'putting something into one's 気' (vital spirit, mental state). The ~ていた form here describes an ongoing past state of worry. The object of worry is left unsaid — recoverable from prior context — which is typical Japanese: drop what you don't need to repeat. 気にする's negative imperative 気にしないで ('don't worry about it') is one of the most common consolation phrases in everyday speech.