The map packs are truly optional. So if you have them all, great, but it won't help in fixing your problem. I believe that since the default maps on the disk are mostly played on live, having them corrupted can prevent you from playing on live. Now, these are the things that you can do:
1. Take out the game and clean it (search for ways to clean disks, I can't really state some here because most of the famous ones (i.e. toothpaste, alcohol, freezing) are so controversial and really with some people but not on others)
2. If attempt one failed, perform it again and try installing it to your hard drive. I believe you go to game library-->halo 3-->install game to hard drive. Once installed, you could go back to your game library and play it from your hard drive instead of the disk. (The game might need to be in the drive though)
3. If nothing worked, than turn off your console and unplug it for at least 5 hours, best if overnight.
Hope this helps

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