Check your water levels, or take a sample to your local fish store and they will test it for you. You may have an ammonia spike. Or some of your other fish may have harassed it to death chasing it around.
It would be helpful to know more, such as what kind of fish it was, who its tank mates are, how big of a tank, and how long you have had him.
clearly you had a fish and it died. so signs of injury have to be shown for your fish to die
it could of died from several things... maybe the water was dirty or it was too warm or cold for the fish... it could even be that he had too much food or too little food and starved
