Calling Ancestry won't help, they'll just tell you to call Nova Development (who the download was actually purchased from), and Nova will tell you to call Ancestry. Bottom line, you need to re-purchase.
When you bought FTM as a download, you were given the option of paying $6.95 for a 2-year "Extended Download", or $9.95 for a backup CD to be mailed to you. If you chose not to pay for those, the expectation is that you would save a copy of the installation files on your own.
If you didn't pay for one of the rescue options, and you didn't save your install file, you can repurchase it as an "Upgrade" for $27.99 by following the steps in this article:http://ftm.custhelp.com/app/answers/detail/a_id/5405
Edit: And when you do re-purchase, either pay for a rescue option or if you are technically comfortable doing so, save your installer somewhere.