If you can't get the judge to change his mind and you can't appeal, you are stuck with the decision.
This is best handled with the assistance of a lawyer.
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You are correct that it is very unusual for a judge to vacate a child support order (and very ripe for appeal as child support can't be changed retroactively). There must have been something very unusual in your case.
If you can't appeal, the only people that could maybe help is the child support agency.
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It is very unusual to have a judge vacate any arrears that are due. You can ask the to reconsider his/her decision, but normally that must be done within twenty days after the decision is rendered. Depending upon when the decision was entered, your only option is to appeal the decision. You will need the assistance of an experienced family law attorney. I offer free consultations, and you can contact me at 414-559-1701. Good luck.