I would call your vet and tell him/her that your cat left and describe the cat's behavior. Vets are very intuitive about animal behavior. Maybe your vet can shed some light on what your cat's behavior indicates. Hopefully your cat isn't too far away. I like the suggestion to leave a bowl of her food outside the door for her with some water. Also, I would put out a sweater you haven't washed yet. Cats are attracted to our scents. Last resort would be to set a cat trap in your yard to catch her. I hope she comes back.