Hello and welcome! So sorry to hear about your fish. You must be devastated to lose that many at one time -- I know i would be!
As Tula mentioned, we need a little more information about your pond. There are so many things that can happen, and most of the time it's a combination of factors. Please answer as many of the questions as you can, and with that information it will be much easier for us to help you and your remaining fish.
- how big is your pond? [dimensions, depth, gallons/liters]
- how long has it been up & running?
- do you check your water for ammonia, nitrites, nitrates, pH, etc.?
- what is the temperature where you are, and is the pond in full sun, shade...?
- have you recently done anything different/has there been a lot of rain or run-off from your yard?
- how big are your fish & how much do you feed them?
- what kind of pump & filter system do you have, and do you have areation via an air pump?
- before the deaths, did you notice anything different in their habits or behavior? were they swimming different or acting distressed like they wanted out of the pond?
- do you have any local predators [animals, birds, snakes, turtles -- all can be a problem]
@Dave 54 might also be able to give local advice.