There are so many reasons that fish get sick, it's going to be impossible for us to guess what might be going on without some more info from you, I'm afraid.
How big is your tank, how long has it been set up and is it cycled? Have you tested the water (and if so, could you post the results) and when was your last water change? What fish are in the tank (species and numbers)?
What have you tried so far?