If you suspect a leak in your pond due to an apparent and abnormal drop in the water level, take action right away. You'll need to isolate the most probable source of the leak first before even thinking about how to repair it. Assuming you have a plastic-lined pond and the water level continues to fall after the pump has been switched off, water is most likely leaking out through a puncture or tear in the pond's lining. You'll need to find the leak's exact location so that you can patch it up.
Switch off the pump, and observe what happens to the water level.
Allow the water to drain through the leak until the water level steadies. If the water level starts getting so low that it might become dangerous for the fish, take them out and put them in a tank or bucket of pond water to keep them safe until you have fixed the leak and the water levels are back to normal.
Examine the pond liner at the water level line for any visible leaks. Also check to make sure the pond liner has not been displaced from its original position allowing for leakage over a side. Be sure not to overlook areas hidden beneath creases and folds in the lining. If no obvious leaks are detected, continue searching.
Put some milk into a squirt bottle, and squeeze some out into the water around the perimeter of the pond. Watch to see where the milk goes. Generally it will flow towards the leak, revealing the source of the problem.
Remove about one more inch of water after the water level has stabilised to make it easier to inspect the liner at that level.