What had happened when Tom wanted to get the attention of the housemaid?

Tom was unable to get the attention of the housemaid because she could not hear or see him.

He did not feel afraid of the maid: as she came nearer, she saw that she was only a girl. To warn her of his presence without startling her, Tom gave a cough; but she did not seem to hear it. She came on. Tom moved forward into her line of vision; she looked at him, but looked through him, too, as though he were not there. Tom’s heart jumped in a way he did not understand. She was passing him.