Perhaps you have taken in a stray of indefinite origin or got your cat that went missing for a few days had come back from some upsetting event, and you pick up on the high possibility that he/she had gone through a terrible ordeal. You wonder just how do you guide them back to their original status of childlike trusting.
Start off by putting the feline in a room of his/her very own and just let him/her settle down... They need both time and space to adjust or readjust to peaceful, safe surroundings...
Cats don’t see isolation as punishment or rejection. A quiet, docile world helps a cat out in a big way. When they get lonely, they will let you know. Until then, let them recover on their own. Of course, have food, water, a liter box and toys in that room for their enjoyment. Also check in on them every so often, if they hide from you, talk to him/her in a gentle voice, but let him/her be.
Replace the bad experience with many good experiences. Create the right responses to reinstate his/her confidence. Let the frightened feline be the boss of that room.
Soon the feline will realize you weren’t the one who harmed them and they will associate you with good things like food, water, a clean litter box, a gentle voice and understanding.
Ever so gradually give him/her time outside of that room to explore the house from this new prospective. Supervise these adventures, sort of police state of affairs with other cats if you have them, be in charge of the disagreeable stimuli that set them off—it may have been a bully cat--and let him/her return to that room as soon as he/she needs to be back in their space again.
Take things slow, be patient and understanding and know your cat will come around.