Illinois allows a final judgment to be vacated on a motion filed within 30 calendar days (not 31, but goes to next business day if 30th day is Sat, Sun, or court holiday) relatively easily. The motion must be filed but need not be heard or decided within 30 calendar days.
Between 31 days and 2 years, a judgment can be vacated on a showing of good cause for not acting earlier and a meritorious defense.
If you were not served or if you are not subject to personal jurisdiction, a judgment can be vacated at any time, without a showing of good cause or a defense.
Note that in addition to handing the summons and complaint to you, or leaving it in your presence (you don't have to sign for it and your refusal to pick it up does not invalidate service), a summons can be served (a) by certified mail, delivery restricted in the case of a small claim, (b) by service on a member of the household over 13 plus mailing and (c) by other means ordered by a court (usually certified mail + regular mail + taping to door).
The return of service is prima facie accurate insofar as it states what the process server could have observed, and you have the burden of showing that it is wrong, with corroborating evidence . The return is not evidence of things which the process server could not have observed, such as the relationship between the person served and the defendant, or where the defendant resides.
We have defended consumers in Cook, DuPage, Kane, Will, McHenry, Kendall, Grundy, Lake, DeKalb, Kankakee, Carroll, Clinton, Winnebago, Bureau, LaSalle, Boone, McDonough, Rock Island, Knox and Whiteside Counties, Illinois