If you used online Turbotax, it does not retain copies of returns that far back. If you used desktop and did not make offline copies, then the only copies of 2015 would have been on your computer. you may want to contact your bank to see if statements from 2015 and 2016 are available. then you would be looking for payments to the Illinois Department of Revenue.
Also, is the notice legit? If the phone number on the notice is the same as the IDR website then it is most likely legit. if not call the iDR use the number on its website.
https://tax.illinois.gov/aboutidor/contactus.html
