The first thing to do is to survey the car's damage as soon as it's safe to do so, preferably with the help of an experienced mechanic. Things to look for include water pooled near the top of the engine, signs of water in the oil and transmission fluid, water in the fuel tank, and signs of flood damage toward the top of the passenger compartment. Any of these could indicate that vital components, particularly the engine cylinders and the electrical system, have experienced serious damage. Whatever you do, don't try to start the car until you get the OK from a mechanic. Doing so can cause even more problems.