Pond liners occasionally leak. Finding the leak is difficult, but patching it is easy.Assuming your liner is rubber or EPDM and most are, you can patch it just like a tire. You can buy a patch online and buy a roll of patching material, because whatever caused that liner to leak could easily do it again. You can also get patching material at an auto parts store or big box store. Since the liner is made of the same material as inner tubes, you can patch the liner the same way.
Make sure the liner surrounding the hole is clean and dry.
After scrubbing it clean, rinse it and let it dry. If you are in a hurry, use a hair dryer, but be sure you are connected to a RCD before you use any electrical appliance around water.
When the area is clean and dry, apply the patch according to the directions on the package. Fill up the pond back with water.Don't forget the de-chlorinator.
If your concrete pond is leaking, there is only one relatively cheap way to fix it. The ground is always shifting, and concrete is freezing and thawing, which makes concrete one of the hardest materials to repair. Clean out the crack and use Plumber's Epoxy to patch it. If that doesn't work, chances are it cannot be patched. Even the epoxy is not fool proof.
If your pond is built from recycled swimming pool liner or PVC or Visqueen and it has a leak, you must start over again with another liner. If your pond is a pre formed hard shell, you cannot repair them after they crack.