I can't play!! WTH

I haven’t been able to play anything in my reach game for at least a week now. No campaign, no firefight, no matchmaking, nothing. And I know it’s not the disk because it works fine on my friends Xbox. I need help,

Sounds like a bad/corrupted update file; I had some similar issues. You can try clearing your cache. On the dashboard, go to system settings, then memory, then press Y and clear your cache. Do it twice. Then turn off your 360 for at least 30 seconds (and optionally power cycle your networking equipment, may help). When you boot back up you will have to re-download game updates, but it will hopefully fix your problem.