Dear pondlady, my fish all died overnight. What should I do?
Pondlady sez: You could try CPR. I'm sorry, I just couldn't resist. First, remove the dead fish and bury them in the vegetable garden, then check your water.
Did you add water yesterday and forget dechlor? That is the most common reason for a total fish kill.
Check these things as well:
Is there run off from a roof or a road or a patio getting into your pond?
If there were children around the past few days, did they throw stuff like an entire can of fish food in the pond?
I would change the water before restocking your pond.