Legal Question in Employment Law in Illinois

Without getting into other legal issues that arise when a company announces a shut down, I will address your specific question about the "severance package" they offered you.

First, the law does not require an employer to provide severance.

Second, the question you ask can only be answered by a thorough review of the severance terms, your job terms, propsect of your continued employment, if you have accepted another job in reliance on the company's announced closure and offer of severance, etc.

You should engage a qualified employment attorney to evaluate your situation and give you specific guidance--something this forum is not designed to do or can do by law.