Start by looking in the court file for the affidavit of service to see how he claims service was made. Then make a motion to vacate the default.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 577-9797 or via email at Eric@RothsteinLawNY.com. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. The above answer is for informational purposes only and not meant as legal advice.
In order for the court to grant a default based on your failure to appear, the father would have had to file an affidavit of service indicating that you were served. Even if you were not personally served, service could have been effected on someone in your home capable of accepting service. Regardless, you have to make a motion to vacate the default. You have one year to make that motion. If you can establish that you were never served and had no notice if the proceeding, the motion will be granted and then you can participate and our forth your defense to the modification petition. File the motion immediately.