If you just want the paper form to prepare by hand:
https://www2.illinois.gov/rev/forms/incometax/Pages/2017/individual.aspx
Make sure you have your 2017 federal return handy. You will need it.
FOR
STATE FILING 2017
I USED TURBO TAX CD FOR MY FEDERAL , STARTED THE STATE BY NEVER FINISHED AND NOW HAVE NEW COMPUTER. AN I REDOWNLOA THE TURBO TAX ON LNE AND COMPLETE FROM MY FDERAL
Don't you have the CD to install it? You can ask if they can give you 2017 download but you better hurry. They are taking down 2017. They only have and support the last 3 years. And even if you can install 2017 you might not be able to update it or download the state program. If you don't mind paying again you might be able to buy 2017 again here,
https://turbotax.intuit.com/personal-taxes/past-years-products/2017
Then if you do get 2017 installed you will need the file ending in .tax2017 to open in the program to add the state. AND After you get the program reinstalled the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW). Enter some basic information, then go to the state tab and download the state program. Then try to open your existing return, go to FILE - OPEN
Contact Customer Service
5am-5pm Pacific Time
Monday-Friday
https://support.turbotax.intuit.com/contact/
What is the Turbo Tax phone number
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
In case you need to just prepare it by hand:
https://www2.illinois.gov/rev/forms/incometax/Documents/2017/individual/il-1040.pdf
https://www2.illinois.gov/rev/forms/incometax/Documents/2017/individual/il-1040-instr.pdf
You will need information from your 2017 federal return.