。
She lost her spark.
Literal
She [topic-は] energy [subject-が] came-to-not-exist.
元気 is one of the first words a learner meets — it's the noun behind the standard greeting 元気? ('You doing well?'). It bundles 'energy, vitality, good spirits, health' into one concept, so 元気がない is the all-purpose way to describe someone who's listless, low, or just not their usual self — the cause could be sadness, illness, exhaustion, or a bad day. The change-of-state suffix ~なくなった adds a temporal arc: she had her usual spirit and then lost it, rather than just lacking it. The 無 here is in kanji, which is slightly bookish — most modern writing renders this expression as 元気がなくなった in hiragana.