A sudden loss of water in a concrete pond means there's a crack somewhere. This can be caused by damage from freezing and thawing or by the settling of the ground underneath the pond, or even by an earthquake. If you've got a leak, most often it's necessary to drain the pond in order to inspect it. The good news is that cracks aren't difficult to fix, and your pond will soon be holding water again.
Drain the pond. This can be done by siphoning the water or using an electric pump. It may be easier to remove plants and fish while the pond is draining if you still have large amounts of water in the pond.
Inspect the concrete. Examine it carefully, as water can escape through even the smallest of cracks.
Widen the crack. If the crack is narrow, widen it with a hammer and chisel. This way your sealing compound will penetrate into the crack.
Clean the area. Use a stiff wire brush to clean off algae, plant growth and any other debris from the crack and the immediately surrounding area.
Fill the crack. Use a trowel to fill the crack with mortar or with a commercial sealing compound. There are dry powder colorants that can be mixed with the compound before applying to match the color of your concrete pond. This way your repair job will be virtually invisible.
Sand the area. To smooth in the area that's been repaired, use sandpaper to bring it down to the original surface.