I am not a big believer in coincidences. If the pond did not leak for seven years, then something has changed.
I assume there was no woody debris in the soil materials that they used for the dam construction? Wood might have rotted out after seven years, allowing a leak path. (Probably unlikely.)
Was your water level this year the lowest water level since the initial filling of the pond? If so, that might be the cause of the change. (More likely.)
That could manifest in several ways. Wave erosion on the dam face could have removed enough material to open a leak path through the core that was previously obstructed. Alternatively, a desiccation crack could have formed in the clay core itself. Clay above the water line typically wicks water upwards and remains moist. However, there is an upper limit to that process.
Do you see any slumping of either the face of the dam, or the backside of the dam? Movement of either face can also allow the core to shift and possibly develop a crack.
Just throwing out more ideas to help you analyze the problem. esshup (and TJ) have much greater knowledge of actually fixing leaks, so I would follow their advice.
Good luck on getting your pond back into proper condition!
I haven't seen any slumping but the backside is covered in thick foxtail so it would be hard to see. Last year it was at the lowest point since it initially filled.
