Based, on the information, it is not possible to understand the reason for her to not be able to walk. There are innumerable possibilities and nothing can be said without understanding the reason.
Next Steps
You should take her to a nearby physician or orthopaedic surgeon who can assess and ascertain the problem. Once a diagnosis is reached, only then can we understand whether it's treatable and what are the possible outcomes.
Health Tips
If a person hasn't walked for a very long time, the muscles and bones in their legs become weaker and weaker with time. So apart from treating the main problem, extensive physiotherapy and supportive treatment is required to restore physical strength. This becomes more and more difficult with the age of the patient. Even then, the patient may be able to move around with support of stick or waker or other
aids.
So, in short, if your grandmother has not walked for a long time, it will be very difficult to get her back on her feet, but only after a proper examination can we be certain of anything.