35 ILCS 5/506.5
For a taxpayer who has received wages from an employer in Illinois, loses or was not provided a W-2 form, is unable to obtain a duplicate W-2 form from the employer, and subsequently obtains a substitute W-2 form from the Internal Revenue Service, it shall be presumed that tax was withheld under Article 7 of this Act in an appropriate amount based on the number of withholding exemptions used to determine the federal income tax withholding for the taxpayer if (i) the substitute W-2 form indicates the appropriate amount of federal taxes withheld, (ii) the taxpayer files a copy of the substitute W-2 form with his or her Illinois income tax return, and (iii) the taxpayer provides a mailing address to which any correspondence or refund, if any, may be sent.
35 ILCS 5/506.5