How do I know if I missed a tax filing from last year?

You should now file the missing state return. There is no penalty if you are entitled for a refund.

If you filed a federal return in 2017 with TurboTax, you can amend your return and add the state return.

Please see the following TurboTax FAQ for instructions on how to amend your tax return in TurboTax.

https://ttlc.intuit.com/replies/3288565

I filed my federal, but can I still file my state tax? I shouldn't file my federal tax again, right? I received refund for the federal already.

No, you do not file your federal return again, but you go through the amendment process to add your state return and send the state return by mail to your state revenue department.