A couple of reasons why you may not have seen your fish recently are:
Pea Soup Algae: Fish actually LIKE green water. It's only us ponders who hate it. If you have green water, it means your pond is healthy. Fish are more comfortable in green water because they don't feel like they're exposed to preditors. If you want to get rid of the green water, see the "Help! My water's GREEN!!" section of this site, or take a peek at some algae treatments on the "Algae Treatments" page.
Iced over pond: In the winter, the fish will hover at the bottom of your pond and you won't see them for months at a time. This is their dormancy period, and although they do not actually hibernate, they are quite inactive and do not feed. Try to net one out though, and you'll see just how active they can be!
Unless you see a floater, you probably haven't killed your fish. Dead fish WILL float. In cooler temperatures it may take a little longer, but eventually, they will show up on the surface of the water, even under ice.
The reason for this is because after a fish dies, it's body begins the decomposition process
If you haven't seen your fish in a long time, are convinced they're all dead, but you haven't seen a body, take heart. They're all probably still in there, enjoying the murky depths. I can't tell you how many poorly installed ponds I've taken out where the owner was convinced all their fish were dead, and we found them all surviving in filth. One pond in particular was only 13" deep, yet the owner had not seen their fish all season and was certain they were gone. I'm happy to report that all 7 of them are now enjoying a new, much bigger and cleaner home!
Now if you have had a heron visit your pond, THAT'S a different story! A heron can empty a pond of all it's inhabitants in a single afternoon. That's one of the reasons we dig our ponds with straight sides, instead of sloping sides like you would find in a natural body of water. Heron and other preditors like raccoons and opossums typically wade into the shallows and wait for lunch to swim by. There are a few instances where a heron will actually dive bomb a pond to get a meal, but they are more likely to stalk their prey. If you have a problem with a heron, there are methods of convincing them to dine elsewhere, but these are smart birds, and they have quite an attitude, so you may have a difficult time keeping fish until you find a method that keeps them away for good. If you have seen a heron in your neighborhood, and haven't seen your fish lately, there is still a good chance that your fish are still in the pond. They may just be hiding. This is a preditory response, and it will take them some time to get over the trauma of seeing a heron or other preditor invading their sanctuary. I suggest you get in the habit of feeding your fish by throwing the food to the center of the pond and not right at the edge. That way, when they see a shadow cross the edge of the pond, they don't rush right over to become a meal.