This chapter of the Chicago Renters' Resource Guide provides an overview of illegal lockouts.
Although your landlord is the owner of the property, you have legal possession of your apartment. The law prohibits your landlord from forcibly removing you from the apartment or from preventing your use of the apartment. If your landlord wants to legally evict you, s/he must terminate the tenancy by serving a proper notice, wait out the time period of the notice, commence an eviction suit, and obtain a judgment. The landlord can then have the sheriff remove your goods from the premises.