If I've used only the desktop version of Turbo Tax to file, can I pull that return in the online version or retrieve it somehow? I do not have the file on my computer

If you used the desktop CD/Download editions installed on your computer, the only copy of your tax data file and any PDF's will be on the computer where the return was created.  TurboTax does not store online any returns completed using the desktop editions.

Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript

For a fee of $30 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf