Sudden is the important word in this question
A cat who has used the tray reliably for months or years and then abruptly stops is telling you something changed, either inside her body or inside her world. Gradual fussiness is one problem; a sudden switch is another and deserves a prompt, structured response. The commonest internal change is discomfort when passing urine or faeces. A cat who hurts in the tray learns quickly that the tray predicts pain and experiments with the bath, the doormat, or a pile of towels in the hope that a different place will feel different. The commonest external changes are a tray moved, cleaned less often, filled with a new litter, or newly contested by another animal, and a household disruption that has made her usual route to the tray feel unsafe. Your job in the first days is detective work, not discipline, because the trigger dictates the remedy.
Separate urine problems from faeces problems
Where and what your cat deposits narrows the cause considerably. Small, frequent puddles of urine, especially with straining, restlessness, or blood, point toward the urinary system and warrant a prompt vet visit; a male cat straining and producing nothing is an emergency that cannot wait until morning. Large puddles in one favoured spot, such as your bed or a rug, more often suggest marking, tray dissatisfaction, or a preference she has developed for that texture and location. Faeces outside the tray with normal urine use can reflect bowel discomfort, litter texture she dislikes for digging and covering, or a tray she now considers too soiled for both jobs. Spraying, where a cat backs up to a vertical surface with a quivering tail and leaves a small amount of urine, is communication rather than toileting and has different triggers again, usually territorial stress. Note which pattern you are seeing before you change anything, because treating all of them as one problem wastes weeks.
Work through the tray and territory changes methodically
Change one variable at a time so you learn what fixes it. Scoop and wash the tray, restore the litter she used happily before if it was recently swapped, and confirm she can reach the tray at all hours without passing a resting dog, a new baby gate, or the cat she fell out with. Add a second tray in the spot she has chosen, which feels counterintuitive but tests whether location was the whole issue; many cats accept the offered tray immediately and the illegal spot loses its appeal once it is simply a tray spot. Clean every accident site with an enzymatic cleaner, since ordinary cleaners leave scent markers she can detect even when you cannot. If a household stressor is the trigger, such as decorators or a visiting animal, confine the disruption rather than the cat, keep her core area stable, and give the routine time to reassert itself after the stressor leaves.
When this sudden change needs professional input
Contact your veterinarian first whenever the change is abrupt, and urgently for straining, blood, appetite loss, vomiting, or a cat who hides and goes quiet alongside the accidents. If health is cleared and the behaviour persists, a qualified behaviour professional can unpick territorial and anxiety causes that are hard to see from inside the household. Bring dates, locations, and what else changed at home, because sudden-onset cases are solved by timelines far more often than by new equipment. Keep recording for a fortnight after things improve, too, since a relapse at the same interval often reveals a recurring trigger such as a weekly visitor. KinsLe Pet Passport holds those dated notes with your cat's vet records in one packet, so the next conversation about this change starts from evidence instead of recollection.