If there are no valuable fish in it now, then pumping it down and getting it dry for examination and repairs would probably be the first step.

If the previous owner was just digging a hole for a dirt supply, then the bottom and sides of your pond probably have little to zero compaction. You might have the proper clay content for an excellent pond if you could just scarify and then compact the bottom and sides correctly.

If the pond did not leak, would you have enough water shed to keep it full? Much easier to have a nice swimming area or pond with a "normal" water level that is fairly stable where you want it.