The Red Cross Barge by Marie Belloc Lowndes

Jeanne Rouannes suddenly awoke from what had been a seven hours' deep, death-like sleep. Awoke? Ah no! As she sat up in a darkness broken by tiny, wraithlike shafts of sunlight, she half smiled, half frowned at the strangeness of the nightmare in the mazes of which she found herself involved.

Instead of being in her blue-and-white room at home, surrounded by all her girlish treasures, and lying in the old-fashioned mahogany bed, opposite which hung a charming portrait, painted some thirty years ago, of her gentle, dead mother, she seemed to be -- of all the most absurdly improbable places -- in the sacristy of the parish church, and sitting up, fully dressed, on a heap of dirty grey coats!