Mildred is a bit baffled, and definitely scared by Montag's behavior of late. He has been questioning her happiness, hiding books in the house, and in the situation you are referring to, he is so angry at the way the world is that he is practically yelling at her. He is questioning everything: his job, their marriage, what happened at the old lady's house the night before. She is fretting, and remarks, "this is the day you go on the early shift...you should've gone two hours ago." When he insists he is sick, she says, "You're not sick," and proceeds to dismiss all of his questions and comments. Of the old lady that burned in her house the night before, Mildred's only comments are "She was nothing to me...she was simple-minded...that is water under the bridge". And the conversation goes like this for a while-Montag being upset about it, Mildred dismissing him like he is a silly child. Then Beatty shows up, and she is worried. She says, "Well now you've done it."
Mildred wants him to go back to work because she wants him to be like he used to be-unquestioning, seemingly happy, not disrupting her life with questions about happiness and life's meaning. If he can just get back to work, then maybe things will be normal again, and normal is the only way that she can function without going over the edge again.