If you find out one day that a creditor has obtained a judgment against without your knowledge, the first thing you need to do is get a copy of the Judgment. The Judgment will give you the case number and the court the case was filed in. In Los Angeles county with that information you can go to the court’s website and see all the documents filed in your lawsuit.
Most likely what happens is that they filed a lawsuit against you, some how got serve against you without you knowing about it, file for a default and submit a 585 declaration to prove their case, and the court entered a judgment against you.
You should go to court and get a copy of the complaint, proof of service, 585 declaration, and the judgment so that you can file a motion to set aside the judgment based lack of notice.
Most court will probably grant your motion because judges want to give you your day in court to defend your case. If the motion is grant, the judgment is erase and you get to defend yourself in court.
The threat of filing a motion to set aside a judgment can also be used as a bargaining chip to settle the case.
Filing the motion is something that you can do or hire an attorney to do.