What happens if bitten by a rattlesnake?

King snakes are constrictors and are not poisonous (unless you meant to ask about a king cobra), so nothing much will "happen". The bite could potentially become infected. As with any puncture wound, it might be worthwhile seeing a doctor and/or making sure your tetanus booster shots are up to date.

A king snake is not especially likely to bite a person without provocation; they're commonly kept as pets, since they're relatively easy to care for and are fairly docile, gentle animals.