Anthropic is spot on....Water temps going from the 90's into the 50's is a sudden turnover....Sad to say, there is nothing you can do now except wait and let critters clean up the carnage. A turnover is a double whammy on the fish, toxic oxygen depleted anoxic water is on top and mixed...fish are both suffocating and being poisoned. Wait till next spring and fish the pond a LOT with small lures and hooks to get an idea of what survived (some really tough, quality fish will survive) in order to determine what corrective stocking you may want or need to do.

On the bright side, the pond will be an awesome fishery again in 2-3 years for a while.