Get the water tested, hun. It's probably easiest just to take it to an aquarium shop or pet shop, but you can buy a kit to test it yourself. Probably worth doing a water change/tank clean after having it tested, just in case the fish were sick too.

PS Take note of which fish were dead, because some are more sensitive than others. My goldfish seem to have the stamina of warriors, but the sucker fish are the first to go if there's something off in my tank.