Agent G just posted the following update in the official Crackdown forum that addresses an issue that some people were running into with the recent Crackdown DLC -

We are aware that the recently released “Crackdown” DLC is causing some gamers to experience a reset of their saved games when they engage in co-op play.  The team has been working extremely hard to identify the issue and provide steps to avoid the problem.  Simply follow these instructions:

  1. Sign-in to Xbox Live
  2. Start Crackdown and receive new Title Update (if you haven’t already)
  3. Begin a Single Player Campaign game
  4. Once loaded, press Start and choose “Quit”
  5. Reboot Crackdown (soft reboot or hard reboot of console)

By following the steps above, this will ensure that your Crackdown experience will be trouble-free – you will be able to enjoy the new title update features, the explosive “Free-For-All” DLC pack, as well as all the goodies found in the “Getting’ Busy Bonus Pack”.  For more details on the contents of the title update, and both the free and premium downloadable content, please visit: http://crackdownoncrime.com/News/DLC+Fact+Sheet.htm

A permanent solution will be released with the game’s next Title Update, which will be available in the very near future.  NOTE: Unfortunately these steps will not restore an old saved file.  There is currently no workaround for this.  If this changes, we will post additional information in these forums in the future.  We sympathize with those who may have lost their saved files.  This was an unforeseen incident and the team has worked diligently to find a solution in the shortest amount of time possible.

Now get back to cleaning up the streets, Agent!