Water has turned a murky chocolate color.
Sounds like a "turn-over", resulting from thermally-statified water in the absence of adequate water-column circulation.
If this sounds confusing, it basically means that anoxic (non-oxygenated) cooler water from deeper areas of the pond rapidly mixed with warmer/oxygenated surface-water (possibly in response to a weather event, cold rain, etc) - producing a net level of dissolved-oxygen that was insufficient for fish-survival. Small/deep ponds that lack natural or artificial circulation seem notoriously susceptible to this scenario.
Lots of discussion on this subject in the "Aeration" section of the forum.
