After a long day at work, she felt completely burned out.

burned out 是动词短语 burn out 的过去分词形式。burn 本意是"燃烧"，out 表示"完全、彻底"，burn out 字面意思是"完全燃烧殆尽"，引申为"精力耗尽、筋疲力尽"的状态。