My campaign progress got reset. Why?

While reading some threads on here, I started to get in the mood for some Halo:Reach campaign (Long Night of Solace, to be exact.), so, I popped in Reach, only to find that all of my campaign progress had been reset.
Why? I had finished the campaign the first week the game came out on Heroic, while signed in on Xbox Live, and I still have all of my game types/films/maps. Has anyone experienced this problem before? It’s pretty frustrating, having to redo all of it. If someone could help me out/inform me of why this has happened, I would be happy to hear/read it.

I too got reset its a bug but not a major one

Not a major bug? Excuse me, but a bug that completely wipes out your campaign progress seems pretty major.
I just hope 343i is ,at the very least, aware of this problem.

It happened to me too, just a glitch Bungie never bothered to fix