Welcome to the forum Chance9888!
As Walt said, copper sulfate can kill fish either through depletion of dissolved oxygen through the dying filamentous algae it killed or through simple poisoning if used in too high a dosage. How is your pond fed? Spring, run off, etc? The fact that you had a heavy rain a few days ago has me intrigued. Sometimes fish kills can also occur due to outside contaminants washed in during a heavy rain.
Irregardless of how your fish kill occurred, what is done is done. It is a shame none the less though. Now is the time to evaluate what you have left and move on. Many members on here have experienced fish kills and I am sure they will share their experiences of what they did and how to bring your pond back to life.
Best of luck and welcome to the forum again...
