If your pond is frozen over the fish will probably die. Then the dead fish will be sucked into the skimmer. Hydrogen disulfide will continue to be produced by decaying matter and will be trapped under the ice. Hydrogen disulfide is toxic in very small concentrations. Normally it will be released into the atmosphere, but it becomes trapped under the ice and lethal concentrations can form.
Either thaw the ice or cut it out to form breathing holes. Don't pound on the ice. Shock waves can rupture the fish's air bladder, killing the fish. I lost a pond of koi when the electricity went out for 12 hours and ice formed completely over the pond surface.