A known issue with Halo Reach is that many times you boot up the game (after signing in) and your campaign progress has been completely wiped out. When this happens, you have to play through the game again in order to be able select different missions. This can be quite an inconvenience, and after writing numerous guides on the game, I've had this happened to me multiple times.
As it turns out, the easiest way to circumvent this issue is by NOT turning off your console while the progress bar is still loading. Doing this, many times, can corrupt the game data and your progress will be lost.
Prior to figuring this out, my data reset nearly twice a week. Since then (and it's been nearly a year and a half), it's never happened.
So just to summarize, the above information will not get lost progress back, but it will prevent it from happening to begin with.
I've seen this question asked numerous times on the developer's home site, and hopefully folks here will find it useful as well.