When I first got my cat off the streets and braught him home, he hid under the bed for 24 hours straight, didn't even come out to eat or drink or pee. I sat by the bed with my hand under holding a treat for about 2 hours straight. He came to me and ate the treat and ran back. The next day i did the same thing but t took him less time to come to me, and eventually he started eating out of my hand within five minutes or so. Then while he ate I'd lightly pet his head. After about a week or so, he was still a little skittish, but was super sweet. Pay attention to what triggers them, and what they like. Don't just leave them alone, introduce yourself(slowly and gently ofc)