If you have used TurboTax Online, TurboTax automatically saves  PDF of all your returns for 7years period ( as long as you are using the same User ID), How to find previous PDF tax returns using TurboTax Online

If you have used a TurboTax Desktop Version, the back up files (.tt**) should be on the computer you had the software installed. Please note for security and privacy reasons we do not have an access to your returns. 

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My files/folders are all missing from my hard drive, all the way back to 2005!