Patricia Wentworth 
The Girl in the Cellar [EPUB ebook] 

She looked into the dead unbroken dark and had neither memory nor thought. She was not conscious of where she was, or of how she had come there. She was not conscious of anything except the darkness. She did not know if time had passed. There seemed to be no sense that it went by, but it must have done, because the moment when she knew nothing except the darkness had changed into a moment in which she knew that her feet were on stone, and that she must not move from where she stood. A gradual knowledge invaded her, and with it a fear that was like the beginning of pain. She did not know how the knowledge came to her. She only knew that it was there. The stone under her feet was a step. It was a single step in a long stone flight. If she were to move she might fall, she did not know how far.