When was she spayed? If she was recently spayed (as is likely since around 6 months is the time most vets recommend to spay a cat) have the vet check the cat to make sure the cat hasn't developed any kind of infection, irritation, or other complication after the surgery. Make sure to tell the vet about the cat's problem, the vet is likely to want to check the cat's urine to make sure that the cat's urinary system is functioning as it should and that the cat's urine is normal. Often right after surgery, a cat is still groggy for a couple of days, and be sore, and more sensitive for several days after surgery, which can contribute to accidents. Young cats often tend to be slow to go to sleep when anesthetized and then remain groggy for a longer time after surgery than adult cats do. When a cat is still affected by anesthetic, the cat may be coordinated enough to get around ok, but be sleepy and slow to react to things, including the urge to potty. I usually confine a cat who has just been spayed or neutered to a cat cage or bathroom with food, water, litterpan, and a bed (a box with a towel in it works fine for a bed) and keep the cat there for a couple of weeks after the surgery. That way the cat can recover more easily and I can also monitor the cat more easily for any potential problems. By confining the cat to a small area, the cat will be much more likely to use the litterpan. If the pan you are using is very tall, try using a pan with less height, a cat may be sore enough after a spay operation that it's reluctant to jump into a tall pan. Cats often will not use a litter pan that is in a noisy place with a lot of distractions. They like a quieter out of the way private place to relieve themselves in, and often a cat that won't use the litter pan in one location, will use it in a more secluded location in the house. Have you changed litter brands lately or used a cleaner or other chemical near or in the litter pan recently? Cats may refuse to use some brands of litter or refuse to use a box because of the scent of some kinds of chemicals or cleaners that have been used around or in the pan. Some cats wait too long before going to the pan, much as young kids do when they don't want to have to take the time to go potty until the urge is right upon them. Multiple pans in different parts of the house can help solve this problem, by making it so the cat doesn't have to go a long distance to reach a pan. I've had a few cats out of hundreds over the years, who would either poop or pee in the litter and do the other business elsewhere and when given a pan of newspaper and a pan of litter, would do one job in the litter and the other on the paper consistently. If you had the cat declawed at the the time she was spayed, soreness of the paws from the declawing may be the cause of the cat's problems with going to the pan too. If the cat happened to experience pain to newly declawed paws, while digging in the litter to urinate, and didn't experience that pain when defecating in the litter, she may have come to associate urinating in the litter with the pain. The solution here is to provide a box of torn up paper towels or newspaper strips for the cat to use until her paws have healed.

My sister just shared with me why her cats stopped using the litterbox. It was the "sand" they used. Their original brands grain size was too large. The grains stuck to the paws and cemented on them. It was painful for the cats. After scraping their paws and letting them heal they timidly started using the litterbox again with the finer grains that wouldn't get caught on the paws.