If seems you used a desktop version. the 2015 Federal and state programs are no longer available from Turbotax, So, unless you kept it installed, even if you had the XXXX.tax2015 file you couldn't open it. offers of the 2015 CD on the web are beta versions which cannot be updated and states are not available. However, if you have that file, you could send it to Taxprinter.com. pay a fee and get a copy of the Illinois return if you in fact prepared it. Taxprinter is not affiliated with Turbotax.
https://www.taxprinter.com/
about the only other resources would be
a) bank or credit card statements for 2016 which MIGHT show a payment to or a refund from Illinois.
b) a federal return transcript for 2015 - doubtful because the IRS also purges old returns. but if it exists from that you may be able to manually prepare an Illinois 2015 return.
