If the funds you received were part of your vested benefits in the plan, you don't have to do anything. The self correction method for an overpayment includes trying to get the overpayment returned to the plan. They also have to notify you that the overpayment was not eligible for rollover. If you don't repay the distribution (adjusted for lost income), the next step depends on whether or not the overpayment was part of your vested benefits. If the distributed funds were part of your vested benefits but were just distributed early, a repayment is not required. They will adjust your balance / benefits to reflect the amount you have been paid and move on. They also have to change their procedures, but that's not your problem.
However, if you were paid an amount that was not part of your vested benefits, if they can't get you to repay it, the company or another party will have to repay the overpayment (adjusted for lost income) to the plan.