The .sync file is in the same folder where the .ftm file is located.
In my case this was: C:\Users\Richard\FTM data
I think that the .sync files are marked "hidden". You have to go into your folder options in control panel and make sure that the hidden files are visible. The option is labeled "show hidden files, folders, drives" on the "view" tab.
I am running Windows 7 and FTM 2014 with the latest patches.
I was able to restore it because I had a cloud based backup at crashplan.com.
You best bet is to restore from a backup of the file before the problem started. My .ftm file was OK. The .sync file had been corrupted.
Note: you may see several versions of the .sync file once you make hidden files visible. That is what I saw and that was the clue. If you don't have a backup, you may want to try to rename the current .sync file and use a copy of one of the duplicate that have numbers appended to it and rename it.
Good luck.