The VERY FIRST thing to do with a cat that has quit using the litterpan is to bring it to the vet to be checked for an infection.
Just like us gals, cats get the same symptoms with bladder infections. It hurts them pee and they have to pee constantly. If allowed to continue, the cat may come to associate using the litterpan with pain and refuse it even after the infection is gone.
I had this happen with a rescued male that had constant bladder infections. He would poop in the pan because he didn't associate that with pain, but refused to pee in it.
I spent 8 years cleaning up after this cat because he was my responsibility and to get rid of him would've meant putting him down.
Meanwhile, I need to move closer to home because my mother is getting frail with age.
I loved Jakey dearly. He died in June of kidney failure caused by the repeated infections. I miss him terribly, but I do not miss the mess or having my house reek of cat pee.
Now that he is gone, I know I can rent an apartment when the time comes to sell this place and move
Squeaky, my other cat, is good about using the pan, but is a clean freak. If pan is not kept scooped, she will kick the urine clumps out onto the bathroom floor!
If she has to "go" and I am cleaning the pan, she will go in the spot where the pan is supposed to be.
I will say one other thing about Jakey. I had a necropsy done on him because he went downhill very quickly.
I found then that the reason he was getting so many infections was because his urethra and bladder were malformed, allowing infection to get into pooled urine because he couldn't empty his bladder.
His urethra was shorter, straighter, and wider than normal. This is just like in a female cat.
Like us humans, female urethras are very short and very wide. This makes it easy for infection find it's way to the bladder. If untreated, that infection can ascend to the kidneys and infect them.
This is what killed Jakey. His kidneys were damaged by previous infections that had ascended, and the last infection was more than his limited kidney function could handle.
So. Your first step is to get her to the vet ASAP. The longer this is allowed to go on, the more likely it is to become a habit.
Your next step is to get a large, open litterpan (many cats prefer them) and an unscented scooping litter. Place the litterpan in a bathroom and put the food and water as far from it as possible. Most cats will not soil where they eat.
Put in a comfy bed that is easily washable. You will have to make time to to spend cuddling and playing with the cat.
If this doesn't do it. Get a "kitty condo". It is a two story cage with shelves in it.
Place a small litter pan (again uncovered) on the bottom of the cage. Place food and water on the bottom of the cage as far from the pan as possible.
The cat will have no way to miss the pan without walking/laying in it. You will find the cat preferring to use the shelves as that gets her farther away from the litterpan.
Place the cage setup in your bathroom. Once the cat is reliably using the litterpan in the cage, open the door and allow her out into the bathroom. Keep the litterpan spotless. Do NOT use Lysol or household cleaners on the pan when you change out the litter. Use a solution of white vinegar and boiling water.
This will kill germs and odors while being harmless to the cat.
Once she is using the pan in the cage while being free in the bathroom. Gate off the entrance to the bathroom. This way the cat can have more freedom, but still go in bathroom to use the pan.
I'd suggest, with a cat like this, keeping the cage and litterpan setup going for her.
It's safe, it's out of the way, and she can do her "chores" without feeling threatened.
If this is Lily, this cat, IIRC your previous writings, has some "issues" as it is. Anything you can do to make life less stressful for her will help with those issues.
Sorry I haven't been around as much. Life got in the way. Hopefully this will help. If you have any questions, feel free to either post them so you get more input from others, or PM me.
Best,
toK