"we remember filing it"
For what tax year? If you think you filed, how do you think you filed--by efiling, or by printing, signing, and mailing?
If you used Online TurboTax, then when you choose that tax year in the section "Your Tax Returns & Documents", what is displayed for "Return status?"
"we received an IRS letter"
Alleging what?--that you failed to file a return? What kind of letter?--there should be a CP or LTR number in the right corner.
