。
She was afraid of speaking.
Literal
She [topic-は] talk [object-を] doing [subject-が] was-scary.
The nominalized clause 話をするの ('the act of speaking') is the grammatical subject (marked with が) of the i-adjective 恐い. 恐い is an older orthographic variant of 怖い (こわい, 'scary/frightening'); both readings are identical. Past tense 恐かった indicates a past emotional state.