。
She was terribly tired.
Literal
She [topic-は] terribly was-tired.
ひどく is the adverbial form of the i-adjective ひどい ('terrible, awful, severe'), made by replacing the final い with く. This conversion turns adjectives into adverbs across the language: 早く ('early, quickly') from 早い, 高く ('highly') from 高い. 疲れていた ('was tired') uses ~ていた not as a past progressive but as a past resulting state — 'was in the tired state' — typical for verbs of physical condition (疲れる, お腹が空く, 喉が渇く), which describe the change-of-state and let the て-form carry the resulting condition.