Just a hunch, distilled in part from the last 2,000 calls for "Help," I would guess that part of the issue in your aquarium is a water quality issue. That is the most common issue in almost ALL of our aquariums, including mine. :)  As superficial as it sounds, partial water changes solve a lot of the problems effecting aquaria, even disease issues by diluting pollutants, removing a certain concentration of disease organisms, getting rid of poisons and enabling the fish's immune systems to work better in cleaner conditions. We should be doing one or two 25% water change a week, more if we have a heavy fish load or if test kits show a dangerously high level of ammonia or nitrites or even nitrates. If a fish dies, if a new fish has not been quarantined (bet on it that it will share something unpleasant) or if we overfeed, those are also reasons for more partial water changes.

Please do a 25% water change with treated (ideally seasoned) water which is about the same temperature as your aquarium. If you had a nearby pathology lab, a lot of money and could get that dying fish over there, they probably could be more specific as to why it died.

This sounds like a lot of work and it is, but you give us very little info to work on. Take a look at http://www.guppylog.com/story/2005/6/24/82111/0134

The first 20 questions will help you describe what your aquarium is like and your maintenance of it. The second 20 questions may help you describe what was going on with that guppy. You may be developing insights into what happened before you ever get back to us. :) If you do answer as many of those questions as seem relevant, we'll give it a shot. We're all amateurs. But the better informed as amateurs we become, the healthier our own fish and the more help we can be to others. :)