We're excited to announce that we've fully migrated the Xbox Forums to their new home, here on the Microsoft Community forums!
As of today, going to forums.xbox.com will redirect you here to the Xbox area of the Community forums. We've moved all of your existing threads over so you can continue and start conversations with the new forums experience.
We sincerely appreciate the feedback you have provided over this transition and can't wait for you to check out the new Xbox Forums!
Read the first two posts on the first link and (although it's too late for you now) the second link lets you know how to properly back up your save file.
It is a common problem. Epic knows about it, but will not do anything to fix it. Unless you backed up your saved data file to a separate drive or the cloud, all of your stats from Gears 3 is gone. Sorry for your loss.