Well, you need to establish what custody rights you have. Hire an attorney, make a court argument through your attorney that it would be in your child's best interests to stay with you and have some visitation rights with her biological father.
Until you act through the court process, though, you are really letting the father control the shots.
The Guru hit the nail right on the head. If he's not returning the child, you'll need to petition the court. Going Pro Se is ill-advised, Illinois Family Law is complicated enough, but court procedure makes it nearly impossible to navigate on your own.
If you can't afford an attorney, contact the McHenry County Bar Association or Prairie State Legal Services YESTERDAY. Time is of the essence, the longer you wait, the harder it will be.
Best of luck & hope this helps.