This is what I would try:
• Download and install Garmin Express on a computer
http://software.garmin.com/en-US/express.html
• Connect your Nuvi to it. Make sure all updates have been downloaded and installed.
• Gamin may have a couple options where you were able to select which state maps were loaded on your system. If Illinois was not loaded, make sure it is.
• If there is no such option, there is another option in Garmin Express that lets you download a complete fresh updated copy of your maps.
Hopefully then you will be able to search the address you need.