The first thing, in most cases, is to exhale. A child will not starve to death if they miss one meal. It's important to keep the big picture in mind and not judge your child's food intake day by day. Instead, think about it week by week. More than likely, it will all even out.
Children go through eating phases.
It's part of child development. One textbook example, and often the most maddening, is a tantrum-inducing refusal to eat anything unfamiliar. It kicks in at about 2 years old and can continue until about age 5. Even children who previously ate anything put in front of them can go through it. During this phase, children often also go on eating jags, where only one or two foods are acceptable.