If you used the TurboTax online editions just sign onto your online account. On the Timelines for your tax returns click on the link shown under the section Some things you can do - Download your .tax file
If you used the TurboTax desktop editions the tax data file will only be on the computer where they were created. TurboTax does not store online any tax returns created with the desktop editions. The tax data file will normally be located in Documents and in the TurboTax folder on a Windows based personal computer.