If your cat has changed her toilet habits and won't use a litter tray, there is likely to be an obvious reason. Even is she has now started to go elsewhere in the house.
Here are the most common causes and some solutions to try:
Dirty litter trays: Cats don't like using a tray if it is heavily soiled.
Solution - Litter trays should be cleaned out completely every couple of days and topped up with fresh litter daily once the solids and clumps are removed.
Put off by litter:
Solution - Using scented litter, deodorants or disinfectants with strong smells may put the scent-sensitive cat off using the tray. Use a mild detergent and hot water or disinfectant specifically recommended for tray cleaning. Cats learning to use the tray initially may need to establish it as an appropriate toilet site and too frequent cleaning may weaken the association.
Wrong type of litter:
Solution - Changing the consistency or type of litter may put the cat off using it. Hard wood based pellets may have been acceptable as a kitten but as cats get heavier there are some that object to walking on the uncomfortable surface. Cats prefer fine grain litter with the consistency of sand with no scented deodoriser. If you want to change the type you use, mix the new one in gradually over a week to gauge the cat's reaction.
Position of the litter tray:
Solution - If the tray is positioned in the open where the dog, children or other cats disturb it, the cat may feel too vulnerable to use it. Instead it may seek a more secure spot behind the television. Cats may not like to use a tray if it is next to a noisy washing machine or tumble dryer. Place the tray in a quiet corner where the cat only has to watch in one or two directions at once rather than in the open or in a thoroughfare. Placing food near the tray will put the cat off using it so place feeding bowls elsewhere.
Type of litter tray:
Solution - Some cats prefer the security of a covered tray whilst others prefer an open tray as it offers more options for escape. If you normally use an open tray it may be worth purchasing one with a lid or vice versa. An inverted box with one side cut out or careful positioning of house plants may provide the necessary privacy. Some covered trays have flaps over the entrance which could be one obstacle too many for the more insecure cat.
Solution - Occasionally a cat decides not to use a tray because it has had a bad experience there. Giving medication or touching a cat whilst it is using the tray may be sufficient to create a bad association. Repositioning the tray to a quiet spot may help.
Early training: Kittens will often soil in the house when they are young if they are given immediate access to large areas.
Solution - When kittens first arrive in their new home they are only weeks away from their original litter training by their mother. Their bladder and bowel control are not as developed as an adult so it is important that the young kitten has easy access to the litter tray at all times. It is advisable to confine the kitten to one room initially with increasing periods of time to explore other areas after a few weeks. Every time the kitten uses the tray it is establishing an entrenched behaviour that will be maintained throughout its life.
If you need any more advice or help with your cat please contact your local vet or vet nurse and they will be able to advise you further.