If she doesn't have a good reason for being late, she'll just have to face the music.

face 在这里是动词，意思是"面对、直面"。 the 是定冠词，特指某种情况或事物。 music 本义是"音乐"，但在习语中常比喻"不愉快的后果"或"批评"。 face the music 是英语习语，字面意思是"面对音乐"，实际含义是"承担后果"或"接受批评"。