I have to warn you that when a fish isn't moving, it's normally very serious. The first thing you should do is a large water change, making sure the new water is temperature matched and dechlorinated. Clean water is the best first aid for fish.
How big is the tank, how long has it been set up, is it cycled and what fish do you have? Have you tested the water at all and, if so, what were the results? Do you know the pH and hardness?