If your fish is still alive it has a problem with its swim bladder. This can happen when a fish is fed flake food all the time, it causes a build up of gas when the dry food is digested and obviously the fish will float to the surface. Cook a few frozen peas, remove the skins and chop whats left up into managable pieces and feed them to the fish. This might not work straight away, but stick with it for 3 or 4 days and hopefully your fish will be back to normal :)