My cat isn't eating. Help!?

My tuxedo cat is only 18 months old. He usually is very good at eating all his food and very playful. The last 24 hours he has seemed to be a different cat. Usually he is meowing at me because he wants more food, but now he hasn't touched his food. I put food out for him yesterday morning and it's still all there. He isn't playing anymore either. He is just laying around the house. Should I just wait it out or should I take him to the vet??? I really don't know much about cats. I've only had him for about 2 1/2 months. I convinced an old friend to give him to me because he was getting abused and I felt bad and didn't want him in that house anymore, but when I got him he was just fine until these last 24 hours.