How do I defend against a court order

by Mae
(Decatur, Georgia, USA)

I am the mother of a now 17 yo son that I apparently lost legal custody of through default. The father applied for custody through manipulating the child protective custody system. He previously worked as an administrator with the agency and had a number of "high level" contacts within the agency.

The agency created a neglect case against me based on the following:

1) In 2005, I lost my job and was unemployed and basically homeless. I did have a child support case thru CSE that was not being enforced at the time. In 4/2006, the father picked up the child one day and would not tell me where the child was nor allow contact.
2) By 6/2006, I eventually became employed and in 8/2006, the father dropped my son off at my employers front door.
3) After I had obtain some living accommodations, the father again picked up my son and took him with him. I did not know where. When the father brought my son back, I was away and he left my son at the temporary living accommodations, alone. The father then had my son, who was 13 yo at the time, call the Department of Family and Children Services Child Protective Services Agency who alleged that I had neglected the child by leaving him alone. We lived in Georgia and under Georgia Law a child at that age can not be left alone. However, the father initially left the child alone. (He used this as an excuse, because he was having marital problems because of the child support case that I had been brought against him.)

4) Needless to say, a case was established by the agency and the child went to live with the father. The agency never called me or visited me again.
5) The agency refused to give me any information on that case and refused to discuss the case with me in person.
6) The father filed for custody using the
agency's "alleged" case of neglect as the basis. We went to court in 2007 and the judge sided with the father, even though no agency representative ever appeared in court to substantiate their case. I had moved out of state by this time, and never received or was served notice of subsequent court appointments/hearings.
7) I moved back to GA in 2009 and in 10/2010 found out about the loss of custody via default, based on a court decision made by the judge on 8/25/10. The judge also awarded the father child support and since I was unaware of the court order, now owe arrears.

My defense is that all other documents (motions, notices, hearings) had been been served by certified mail or law enforcement. I never received any notices.
In 11/2010, I checked the court records, and find out that the income figures used to calculate the child support are not based on the income documents that I faxed to the court after the 1st custody hearing in 2007, nor was the income reported by the father correct (he reported substantially lower income). Can I file a motion to vacate that order at this time or challenge the order by some other legal means?