Were you using Sync with a 2nd device? If so, your data should still be on that device and Sync should be able to get it from there.

But if you used Sync with one solitary device and then installed a new operating system on that device; and then expected to use Sync to "recover" you data and that didn't work your data may be gone. Many users who have attempted to use Sync in that unapproved manner lost their data when they changed their Firefox Account password before their data was re-Sync'd back to that device. The password that was being used when the data was sent to the Sync server is the "key" that is needed to decrypt that data, a different "key" won't work. So when the account password is reset the Sync server clears any data that is sitting there.