Determine the exact problem by allowing the water to drain out. When a leak is present in a pond dam, the water level tends to drop fairly quickly up until the point that it falls below the point of the leak. By monitoring the water levels, you'll be able to see when the water ceases to drain out rapidly and will therefore know the level of the leak in the dam.
Visit your local agricultural supply company to obtain bentonite or a chemical pond sealant. Unless your pond dam is severely compromised (which would be very clear as water would be visibly pouring out the other side of the dam), an application of these substances according to manufacturers' directions will solve the problem.
Spread the pond sealing substances all over the above-water portion of the dam, as well as in the first 3 to 4 feet of water as well. Whether using bentonite or chemical sealants, the process is the same: once wet, these additives swell many times, filling gaps and crevices in the otherwise porous pond bed.
Monitor water levels during the following weeks to determine whether the problem is sufficiently solved. If leaking persists, you may have to address structural issues as the leaks could be the result of a weak dam being slowly moved by the force of the water; check the height/width ratio of the dam if this is the case. A dam should be at least twice as wide as it is high, and if yours is less than that you'll need to drain the pond and add additional fortification to shore up the structure.