I will try to get you started though. There are a lot of things that could have gone wrong and you really don't give us enough info to make even a wild guess. How old is the pond? How big is the pond? Did it freeze over this winter? Did you have an air bubbler going through the winter if you had ice? Did you have a hole in the ice? How many fish do you have in the pond? What kind of filter do you have? Do you have rocks on the bottom? Do you have a waterfall? Do you test your water? What kind of fish are they? How big is big for the fish? Is your pond close to a road? Was your pond cleaned out at the end of the season?

Older ponds are usually more stable than new ponds.
The dimensions are important. Most pond fish need a depth of 3 feet at least in some part of the pond.
A pond that freezes over needs different care through the winter than a pond that doesn't freeze.
Too many fish in the water makes less oxygen available. Big fish are affected before smaller fish.
Bubblers provide oxygen to the water especially under an ice cover.
A hole in the ice allows bad gases to escape.
Filtration is important. Debris, gunk and fish excretement in the water creates poisonous gases that can kill your fish.
Rocks that cover the bottom trap debris and excretement that rots and creates poisonous gases, in the opinion of many posters on this forum.
While a bio filter stops working at lower temperatures a mechanical filter will continue to remove debris and provide at least some oxygen.
A waterfall will add some oxygen if it splashes.
Testing the water for nitrates, nitrites, ammonia and PH tells you if the water is unhealthy. A bad reading on any of these warns that your fish may be in trouble.
Comets and Gold fish are often more hardy than Koi. Comet hybrids like Sarassas are less hardy than plain Comets. Plain white comets seem to be less hardy. Long finned fancy comets seem to be less hardy.
I have a fair selection of fish sizes. Most of my comets seem to stop growing at 8 to 10 inches except for the fancy long finned one which will grow to 12 inches. Koi get much larger. I have one that is about 15 inches and they can grow to 2 feet long.
Salt for road use is not the same as the salt we sometimes add to the pond water.
Getting the pond clean before the weather gets bad is important maintenance.

The possibilities for the frogs demise cover a lot of territory, too. It is possible that it was just its time to die. That is a possibility for the fish, too.

I keep thinking of other possibilities, too. What can you add?