Lots of people complaining on the cod forums about it. Look here....
http://forums.xbox.com/17843809/ShowPost.aspx
EDIT
I think this is importnant to note, dont play a game til you have turned the box on and off..........
Do NOT get upset and start playing to make up the ranks.
What has happened is while downloading your personal settings, your old data became jumbled with your current data. So, if you play you will over overwrite what your current rank should be.
- How can I get my rank back?
Just turn your xbox on and off ... or go back and forth between multiplayer and single player (whatever you can do to make Xbox Live show the screen to download your personal settings). Doing this will make the servers continuously search for your data, eventually it will find the correct data and fix the problem for the time being.
- I've done what you said, but my rank still isn't coming back, what's wrong?
#1 - Normally these problems do not sort themselves out the very next time you download your settings. For quick access purposes, servers will set apart your information for X amount of time so it doesn't need to search for it time and time again for only short leaves from playing the game. One of the best ways to fix this is to shut off your xbox for a short period of time, maybe 5-10 minutes ... then start up and go to xbox live, see if it can download the right information, then rinse and repeat until it is fixed. Repeatedly just backing out of xbox live and going back in will rarely fix your problem, the servers need time to reset your data and put it back into "storage".
#2 - In RARE cases, some people might actually lose their information permanently. It is inevitable that any system can lose packets or get corrupt files but it is extremely rare. I am not the one to tell you that it is the time to give up and re-level, that is your place to make the final call but if you feel that you fall under this category just remember... when you play your first game, it is permanent, that will now become your new level to build from.