What happens when due process is violated by the state?

Timing here is important. You have ten days to request a rehearing and if you were not notified of the hearing, you may have a good opportunity to revisit the order. You also have thirty days to file a Notice of Appeal if you are in a district where a re-hearing is not a prerequisite and you have missed the ten-day deadline I cited above.  This is a very peculiar situation where, as you said, no party received notice of the hearing. You should have a local family law attorney review your facts and timing to ensure you are accurate