Sudden fear, phobia

Have you thought of desensitizing?

There is a good article here about it, in the same way- humans re not able to overcome a fear until they know there is no reason to fear, cats can do the same through desensitizing or "countercoditioning" basically, exposing them to the "fearful" item and associating the noise with something good.

I did this on Smokie with thunderstorms.  He used to be TERRIFIED of storms, but slowly and surely- I had special toys just for that time, and special treats just for thunderstorms. Now, he associates thunder with pleasantries.

http://maxshouse.com/behavior/desensitization_and__countercond.htm

They list seven steps:

1. Safety first: because a fearful cat can quickly become aggressive, precautions must be taken before beginning a treatment program for fear aggressive cats. Ideally, exposure to stimuli should be sufficiently positive and gradual so that no fear is exhibited. The cat should be entirely separated from the fearful stimulus except during retraining sessions. If necessary, a restraint device such as a leash and harness works well for some cats so that they can be interrupted from a distance and moved into a separate room until they calm down. Other options include using a large blanket to cover and wrap the cat and carrying it to a separate room until it calms down. Some cats may be successfully interrupted with a spray from a water rifle, citronella product, canister of compressed air, or with a noise device.

2. Stimulus identification: each and every fear-eliciting stimulus must be identified, including people or animals, and in what situations fear is likely to arise.   For some cats, fear may be generalized so that all strangers or other animals lead to fear and anxiety, while for some cats the fear may be only to specific stimuli such as a particular family member or other pet in the home.

3. Stimulus control: some method of controlling exposure to the stimulus must then be devised, so that a safe and effective desensitization program can be implemented.  Avoiding interactions with the stimulus is the safest approach, but improvement cannot be made without exposure to the stimulus. Using a leash and harness, a crate or separation across a doorway can be used to initiate exposure exercises with animate stimuli (e.g., people, other cats) while videos or audiotapes might be useful for auditory stimuli.

4. Stimulus gradient: each stimulus will need to be presented along a gradient from low (least fear evoking) to high (most fear evoking). To develop a gradient you will need to determine which situations, people, places, or animals are most likely to evoke fear, as well as how to minimize and control these stimuli for retraining. For example, a cat that is fearful of strangers may be most fearful of the approach of young children, and least fearful of adult visitors that ignore the cat.

5. Reinforcer selection and assessment: for most cats, special food or treats are likely to be of highest appeal, so that these reinforcers should be identified and saved exclusively for desensitization, counterconditioning, and reward training. By depriving your cat of these rewards until the training session, the reward may then be a strong enough motivator to overcome low levels of fear when exposed to the stimulus. Favored toys, catnip, and even short periods of affection may also be effective for counterconditioning, if they are saved exclusively for the exposure sessions.

6. Pretraining: using a ‘learn to earn’ type program, many cats can be trained to have a positive and predictable response to commands or phrases. In this program, prior to giving the cat any reward or anything of value, the cat must first exhibit an appropriate response. A few basic phrases or commands, such as ‘come’ or ‘feeding,’ ‘play time’ or ‘go to your room,’ could be learned by the cat if the commands are always followed by a reward. In addition, a positive association needs to be made with any new control technique. If the cat is to wear a harness or be locked into a new confinement area, food, treats, or play should be provided.

7. Other techniques for reducing anxiety: the use of pheromones and drug therapy may also be useful at reducing anxiety so that the behavioral retraining program can have faster or better results.

Does he appear to be in pain?  Sometimes a loud noise he may have been used to could have been happening when something happened and he hurt himself... Now he associates loud noises with pain.  I would take him to the vet to rule out any medical problems first and then go from there.  Maybe The Calming Collar and Feliway plug ins at the same time would go a long way to helping him and you?  But first thing first, take him to the vet.  Cats are good at hiding pain and injuries.  You may not even know he's injured!

Your cat may have been spooked by something you didn't see (like a cat outside, for instance). 

One of my cat got stuck in a plastic bag last week, BOTH cats totally freaked,

and they're still not 100% back to normal.

They run from the noise of fans, vacuums and my pressure cooker (this one actually induces long term under-the-bed hiding ).

Does he still play, eat and use the litter box the same as before?