If you were supposed to mail in your state return and you never did then your state return was not filed at all and you would not receive a refund until the state is able to process your return.
You can get access to your 2015 returns if you upgrade to PLUS or higher. Go to My TurboTax > Tax Timeline > Load My Tax Timeline > Click on Tax Year 2015. You can print out your already completed 2015 return and mail it to the state.
