There may be a number of reasons why a cat may be scared, it can suddenly occur in a usually friendly and laid back cat, or it can be their nature in general.
Lack of human socialisation from kittenhood is one, individual personality (some cats are just more shy/nervous than others), environmental (a trip to the vet, a stranger in the house, being given a bath), past traumatic experience (being abused by a human, chased/attacked by a dog) a sick or injured cat may also be scared.
When a cat is scared, for whatever reason that may be, he is more likely to lash out in fear. Which means you need to be careful. A scared cat is capable of inflicting a great deal of damage.
What are the signs of a scared cat:
Being able to read a cat's body language is important to determine his current mood. A scared cat may exhibit the following signs.
Dilated (large) pupils
Crouched down low
Obviously, there are different levels of "scared". A cornered feral cat or an injured cat will be very frightened. Other situations may be less severe, such as your cat just being a little scared of newcomers to the house. You have to assess each situation and cat on a case by case basis.
It is generally accepted that feral cats who have had no human contact after the age of seven weeks can not be tamed. I am sure this isn't always the case, but by this stage in their development, they have developed a fear of humans.
Is the stress temporary?
Temporary stress is easier to work with. Say your cat is scared of going to the vet, you can work towards alleviating this stress by doing the following:
1) Finding a veterinarian who does house calls (although this is only going to work for minor ailments, routine vaccinations etc. Your cat will still need to go to the surgery for more serious conditions or medical tests.
2) Slowly getting him used to the veterinarian. This may include a light sedative prior to a visit (if he's really scared). Pam Johnson-Bennet suggests taking your cat to the veterinarian just to say "hello" (I'm paraphrasing here). Instead of only ever showing up at the vet for medical examinations or treatments, put your cat in a carrier, drive him to the vet and just pop in and say hello, without actually having your cat examined.
Is the stress due to sickness or injury?
A sick or injured cat may feel stressed, especially if he is in pain. The need for medical treatment is obviously a necessary outcome, but you must be careful with a stressed and injured cat. Even one who is normally placid and compliant can lash out under these circumstances.
If possible, cover your arms and legs with long pants and a long-sleeved top, wear gloves and if possible glasses or sunglasses. Approach the cat calmly, crouch down to his and talk soothingly to him.
Give him a calming scratch behind the ears (if possible).
If he is acting aggressive, place a towel over him and wait a minute or two. This "should" calm him down. Once he is calm, slide the towel under the rest of his body and carefully place him in a cat carrier.
If he is not acting aggressive, just scared, pick him up and place him in a cat carrier.
What to do if your cat is scared of you:
We can work with them to bring them out of their shell, but I have certainly found that it's not always going to be possible to turn a shy cat into a friendly and outgoing cat. I have had two cats like this. One was scared and vicious, the other was timid. We worked hard to build trust and confidence in both cats, and to a degree, it helped, but they remained quite shy and frightened all their lives.
Food and play therapy can go a long way in building the trust of a scared or timid cat as you begin to associate yourself with "fun and pleasant" things. If you have a timid cat, try to lure him out with small cat treats. Don't ever force the issue, just sit close by and offer a treat, as he comes closer to you, give him more, until he is close enough to gently touch and stroke.
Another/similar method to the above is to coax him out with a cat toy (generally a wand type toy is best for this), get him closer and closer to you. Hopefully, he will be distracted by the toy and forget his fear.
Scared of strangers:
I think to a degree, we should accept our cats for who they are and that may include a fear of strangers. Obviously, it is not ideal if your cat is afraid of the veterinarian as he will need to see the vet every now and then for check-ups, vaccinations and medical treatment.
However, if in the home, they are not keen on visitors you have two options. Build up the trust of your cat by having your visitors play with, or give your cat treats, or just accept that he's shy and leave him to it.
I think it is important to always provide your cat with a safe haven where they know they can retreat to without people bothering them. That may be a dark corner in the wardrobe and or a cat tree where they can observe the world from a safe height.
Frightened new cat:
If you have just adopted a kitten or cat, it can take him a little time to adjust to his new surroundings. Kittens may be missing their mum and littermates. This is perfectly normal and usually, settles within a few days. You can help the process along by offering him a safe and secure place to sleep with a warm hot water bottle (wrapped in a towel to prevent him getting burned), and try placing a ticking clock close by which can remind him of mum.
Adult cats can take a little longer to settle into a new home, so give it time. Again, I recommend you use play and food therapy to win his trust and confidence.
Change in behaviour:
If you have an otherwise friendly cat who suddenly becomes scared it is time to take him to the veterinarian for a check-up as it can be a sign of an underlying medical condition.
Helping your scared cat:
Cats secrete feel good pheromones from their cheeks and paw pads, these serve to reassure your cat and calm him down. There is a product on the market known as Feliway which is a synthetic form of this pheromone. Using this around the house can have a calming effect on your cat.
Bach's Rescue Remedy is a flower essence which can help calm a scared or anxious cat. Add four drops to your cat's food or water.
Play and food therapy should be used to build up trust and confidence.
A safe place to hide/retreats to if your cat is overwhelmed or just needs a place to escape to. This can be a corner in the wardrobe, an enclosed bed/cat carrier and or a cat tree.
If the above methods don't help, speak to your veterinarian about drug therapy. This should not be a long term solution, but used long enough to help build up trust and confidence in your cat using the above methods.