I got a letter from Franchise tax board in CA stating I didn't file my taxes in 2018. Where is the proof I did.

[gamma2018]

Where is the proof my taxes were filed, I did it through Turbo tax.

[DoninGA]

If you e-filed your 2018 state tax return and it was accepted by the state, there will be a State Postmark Report that will have the date/time the state tax return was e-filed and the date it was accepted by the state in the PDF download.

To access your prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Download/print return (PDF)

[VolvoGirl]

And when you log into your account you should also see the status and if it was Accepted or Rejected, Started, Printed, Ready to Mail, etc.  what does it say?

Maybe it Rejected.  Do you still have last year's emails?  You should have 2.  The first email only confirms the transmission.  The second email says if the IRS (or state) Accepted or Rejected your efile.

Did you have a refund or tax due?  If you had a refund, did you get it?  Did the letter say they didn't get your tax return or maybe they didn't get a tax due payment?