、。
She isn't her usual self today.
Literal
She [topic-は] today, energy [subject-が] does-not-exist.
元気がない ('not feeling well, dispirited, low energy') is the negation of 元気がある — using the が-marked subject pattern of ある. Distinct from 元気じゃない ('not energetic'), 元気がない carries a softer, more concerned tone — something is off about her vitality.