I would follow wilders advice to the letter, and if you have one, put in an airstone for now, as BettaMomma said, that will help with depleted oxygen.
Do your water change, then get your water tested.
The dead fish could be the cause, or be an effect of bad water. Do you do weekly/bi-weekly water changes? Lack of maintenance can lead to a high concentration of nitrAte, not as toxic as ammonia and nitrIte, but still bad.
The water change and airstone should show an immediate improvement with your fish, but don't think thats it, still test your water.
Also, is your filter output still strong? If it has become clogged, then the agitation on the surface will have decreased - along with the oxygen content of your water.
Jon