Sudden Death?

It is not unknown for fish to just die like that, and most of the time we never know the cause. If a fish was sick or had been bullied there is obviously a reason, but not for a healthy fish to just die. It is something we have to learn to accept happens every now and then.

Checking the water conditions like you did is the first thing to do, and even if nothing is amiss it is usually a good idea to do a water change whenever a dead fish is found.