This happened to me too. It sucks, I know. You have to unlock all the missions again. I had beaten the game on legendary, and after it got reset, in my service record it still said legendary for "highest difficulty completed".
When this happened to me, it also reset my settings (brightness, controls). You should make sure you have your preferred button layout and other settings.
*Edit: I don't know what causes it. It has happened to me twice in one week, but its been fine since then (2 months ago).
[Edited on 11.02.2011 1:38 PM PDT]