There could be hundreds of reasons. You don't mention the reason for and type of surgery. You need to give more detailed symptoms and complaints. Also, her age is a very significant factor in previous surgery outcome, future surgery relevance and amount of any potential improvement.
I appreciate your concerns about your grandmother and I agree that the pain needs to be investigated. Usually knee locking and pain after total knee replacement is due to formation of fibrous tissue around the patella. Rarely, it is due to blocked or pinched popliteal artery that runs along the back of knee.
The various other causes of knee locking are torn medial meniscus, mis-aligned knee cap, osteoarthritis, knee arthritis, torn lateral meniscus, osteochondritis dissecans, recurrent patella dislocation, chondromalacia patella, knee alignment problem, knee fragment or knee cartilage disorder.
Since pain intensified after a fall, maybe she has injured a ligament. Please discuss with her doctor. It may be a good idea to do a MRI, if that is possible. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.