If it's a accurate pond frost can do that and you may accept to abandoned it to find the problem/crack, possibly put a liner in. If it's a liner pond then let the baptize bead and area it stops bottomward you will acquisition the leak, normally area some beastly has clawed it's way out, there are patches for liners.

The problem with any leak is that if you let the water drain down, the leak may not necessarily be where the water stops. It could be below that, but the pressure may now be sufficiently low to stop it leaking further. Also the height of the water table can affect the levels to which a leaky pond will fall.
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