I have Call Of Duty 4, and in one match, I did bad. Then all of a sudden, it said it will take a couple million experience points to level up to Private, when I was already a Corporal, and now it won't let me go online. How can I make it work?
What I have to guess is that you ended up in a modded lobby.
The bad news is that this was always a big problem with the 360 version of COD4 and it's actually gotten worse. You'd probably run into another negative XP bug after a dozen matches again.
The WORSE news is that Activision stopped taking requests to fix this as of 2014.
The only thing you can do to sort this out is find someone who's in a modded lobby, ask them to invite you and use the mod menu to reset your XP. Search around you'll find several forums with dedicated threads to help you out(I won't link them for obvious reasons).