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.
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.