。
She was very excited.
Literal
She [topic-は] very was-being-excited.
興奮する ('to get excited') is a Sino-Japanese suru-verb, and like other change-of-state verbs it pairs with ~ている to describe the resulting state of being excited. The past form 興奮していた then captures 'was in a state of having gotten excited' — past progressive in feel. Compare 興奮した which would be the punctual 'got excited (one moment).'