A cat that stops using the litter box is telling you something. Most cases come down to pain, a box that does not suit the cat, or stress somewhere in the house. Any sudden change has one order of operations: vet first, box second.
A male cat that strains, cries, or crouches in the box repeatedly without passing much urine may have a urethral blockage. This is a genuine emergency: the bladder can rupture within 24 to 48 hours and the cat can die. Go to a vet immediately, at any hour, rather than waiting for morning.
Other medical drivers include cystitis, bladder stones, kidney disease, diabetes, arthritis that makes climbing into a high sided box painful, and an overactive thyroid. A urine sample and blood work usually identify the cause.
Do not wait to see whether it settles. A cat that urinates outside the box for three days may have a treatable infection. A cat that has stopped passing urine at all has hours, not days, and blockage is far more common in males because their urethra is longer and narrower.
One box per cat plus one spare is the standard recommendation. A box should be about one and a half times the length of the cat from nose to tail base, which means many covered boxes sold for cats are too small for a large adult.
Fine, unscented clumping litter suits most cats. Scented litter, crystals and plastic liners put some cats off completely. If you need to change litter type, mix the new one in gradually over a week or more.
Place boxes in quiet spots with two open exits where the room allows. A box next to a washing machine, a boiler, or a doorway everyone uses collects avoidances without you ever seeing the reason.
Scoop solids at least once a day, twice in a busy multi-cat house. Dump and wash the whole box with unscented soap every one to two weeks, then refill to the same depth so the feel stays familiar.
Never use ammonia based or heavily perfumed cleaners. To a cat, the residue smells like another cat marking the spot, which invites spraying or avoidance. Hot water, mild soap and a full dry before refilling is enough.
If the box is cleaned less often than the cat uses it, the cat will find a cleaner surface. That surface is usually a bath mat, a laundry pile or one carpet corner, and once a habit forms there it takes weeks to undo.
The most common non medical trigger is another cat. A cat that is ambushed on the way to the box will avoid it entirely, and you may never witness the ambush because it happens at night or behind a door.
Household change counts too: a new baby, a moved box, building work, a new pet, or a visitor who smells unfamiliar. Cats want routine and a sense of control, and the litter box is where the loss of it shows up first.
Watch for guarding, staring, blocked doorways, and one cat that always leaves the room when another enters. Spread food, water, beds and boxes across separate rooms so no cat has to pass another to reach what it needs.
Write down how often the cat goes, whether it strains or cries, the amount passed, where the accidents land, and any change in drinking, appetite or weight. Photograph the accidents and the colour of the urine so nothing is lost in description.
Take a fresh urine sample if you can. A clean, empty tray with a non absorbent litter or a sample kit from the clinic often saves a second trip and a second fee.
Cleaning matters for retraining. Use an enzyme cleaner, because ordinary detergent leaves the scent marker behind and draws the cat back to the same corner. Block access to the spot for a few weeks while the box habit rebuilds, and put a box nearer that corner.