(Haven't read the above posts, so hopefully this isn't repeating)
Well, I mod Halo maps ALL THE TIME! I have a folder in my documents called "Halo Original Maps" which contain all the default map files I copied over from the "Halo>MAPS" folder. That way I can replace them and can't screw anything up if something happens. (Just a suggestion for the future if you like modding maps) Now on to YOUR problem. Well, the only things I could think of would be getting the map from someone over Xfire, and reinstalling your game. They both work fine and reinstalling won't make you put the key in again so it will only take a minute. So, do whatever you want. But don't try copying bloodgultch.map and sidewinder.map from your Halo CE maps folder. Even though there is no difference in the maps themselves, the map files are different and Halo will think it is corrupted and won't run at all until you reinstall the game.
- Lucky