Litter Box Problems - Cat Peeing Everywhere
10 Reasons Why your cat or kitten stopped using the litter tray

Cats can stop using the litter box for a great number of reasons and there is not always a simple answer to the problem. Below are 10 reasons why your cat may have stopped using the litter box that may help you resolve the issue.

Your Cat may have a medical problem

Urinary Tract infections are a common cause of a cat peeing all over the house.
Often a cat will inappropriately urinate because of a medical condition. If you have problems with a cat not using his litter box please get him or her checked by a vet. Your vet can examine your cat for lower urinary tract diseases to rule out medical conditions which could be causing the inappropriate urination problem. For example a painful case of cystitis or urethral blockage may have your cat associating the pain of urination with it's litter tray and it will therefore avoid it's litter tray

Read these articles for more information
Pet Pee & Poop Problems - Discussion
Cystitis in Cats can cause inappropriate urination 
Urinary Tract Infections in Cats 

Diabetes

Diabetes in cats can cause increased urination and maybe your cat doesn't make it to the litter tray every time.

The Litter Box is dirty

Cats do not like using dirty litter boxes. Make sure you change the litter daily and wash the litter box with hot soapy water at least every few days. Scoop out soiled litter regularly in between.

Your cat may not like the brand of litter you have chosen
 

Maybe you have switched brands of litter or bought a completely different type of kitty litter. Cats are creatures of routine. If you wish to change the type of cat litter you are using then you can gradually introduce the new litter by mixing it with the old a small bit at a time until the new litter is accepted. Often cats don't like the same nice smelling kitty litter that we might like. Avoid scented kitty litter.

The location of the litter box is not to your cats liking

Don't put the litter tray in a busy location or near a noisy washing machine for example.  Put the litter tray in a quiet easy to access location and don't forget they like privacy too! Cats do not like to use the toilet near to where they eat. That's not surprising... neither do we! Move litter boxes away from feeding areas or where their water is placed. Do not put the litter box in a room or basement where the door may be closed and the cat cannot access it.
 

Your cat is spraying urine not urinating. When cats spray urine it is not normal urination like when they need to go to the toilet. It is territorial and can be stress related.

Male Cat Spraying Problems? The causes of spraying and how to prevent it

Your Cat may be stressed

Cats suffer from stress as well as humans. Cats are creatures of habit and thrive in a consistent and familiar atmosphere. When their 'comfort zone' is disrupted it can often result in stress which contributes to inappropriate elimination. 
Read this article on Feline Stress Common causes, management and diagnosis of feline stress
 

Your cat may not know where the litter box is

This is common in kittens who are busy playing and may forget the location of the litter box. Once you have found a suitable location try not to change it.

Multiple Cats

Some cats do not like sharing their litter box with other cats while some don't mind at all. Maybe your cat is one of the former and requires his own personal litter box. Multi-cat households pose numerous problems. Cats are territorial and by instinct they need to have their own space. In a multi-cat household this is often impossible and stress levels in the cats are raised. One cat will pee somewhere to show that this is their territory then along comes the next cat and pees over the top to show that is their territory and so it perpetuates. Pooping can also be a problem with cats in a multi-cat household defecating outside the litter box. It's sad to say but often there isn't a solution to urine and defecation problems in a situation where there are multiple cats living in a confined area. You have the choice of cleaning up or reducing the number of cats.

De-clawing

De-clawing is still legal in some countries. If you have had your cat de-clawed then it may be painful for him to scratch in the litter.
Read this article about declawing

Don't add more stress

If you catch your cat urinating on your carpet or floor, gently and quickly move him to the litter tray. Don't punish him as this may increase his stress. Try to reinforce positive behavior. Punishment only causes more stress.