Must be that you are taking care of someone's cat. Don't worry, it just takes some time. Cats are very sensitive animals. They don't like to have to change their living surroundings. They got really freaked out and stressed when moved to a new place. They don't wanna eat facing a new face who they don't recognize yet. Try not to give him/her too much attention, just show him/her where the water/food is. Leave them alone, you will only find empty dishes after a day or two.