There is a patch for the US Xbox version to fix an issue over-writing save slots. The patch for non-US versions has not been released yet.
Are you getting a consistent loading screen problem?
Did you try exiting back to the dashboard and restarting/reloading?
I had my Xbox lockup just before a loading screen once (as the screen faded to black), but turning the console off and restarting fixed it (I had a save a minute earlier). The Xbox had been on all day, so may have had heat issues.
