First step would be to do your research and find out what court issued the judgement against you, if any. Then you would need to see what process they used to serve you and if it was proper. (There are a lot of Sewer Services out there that didn't do what they are supposed to do in regards to process serving).
I'm assuming you would start in the county court where you used to live. The old address that they just sent the paperwork to.