Are you softmodded or chipped?
For a softmod, run the game you used to install the mod. Install it again or get FTP access and FTP the back ups on your PC to the Xbox. If you don't have backups, I strongly suggest you back up everything as soon as you get access. This can be fixed with Xboxhdm and your eeprom.
If the HD ever dies and you don't have the eeprom, you will need to chip it.