The TurboTax online editions can only be used for one tax return per account. When your son's return was completed on the account it overwrote your tax return and your return is gone.
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
