I am sorry to hear your cat won't come back inside. My name is Rebecca. I am a veterinarian and will do my best to help.
How long since she got outside and won't come back? Can you catch her at all?
I would be concerned. There are many dangers outdoors, and depending where you live. In my area, we have many coyotes and cats usually do not last long if they go outdoors. Other predators, infectious diseases from other cats, dogs, cars, and people with guns are all threats to cats that are outside. Is she sticking to your yard? That at least would be safer.
I assume you are feeding her, and she comes around for food? I would start putting out a cat carrier, with a bed in it, and the door open, and only put her food in there. You may have to start putting the food just inside the door but then gradually work it toward the back of the carrier. When she starts going in and out regularly, she may even start to use it as a bed or a nest. One day if she is in it, you simply close the door and you can get her back inside.
She may be fearful of the carrier at first, but if that is the only place she can get food, and if the carrier does not move at all, she will become accustomed to it.
Let me know if this is helpful and what other questions I can answer.