Unknown CD scratch. L4D. Cannot load main menu...

Like so? It's worked for me before...
But how serious is it? Photos?
EDIT: And is it a cd or a dvd?

Try loading it onto your HDD, and then seeing if it'll work. That's the only way I got my RE5 to stay working.

I'd take it to your local music shop and have them buff it out with a specialized machine. Its like 5 to 10 dollars and it works just dandy. Best thing, if it didn't work, you can return it for a rebuff for free. Or at least thats what they do with my place.
I bought Madden for my brother but the DVD didn't work, so I buffed it and it was a cheap but welcomed present. YMMV

It looks like your 360's laser diode got to close to the DVD, you really just laser etched it man. Toothpaste did not work when my GoW disc did that, maybe you will be lucky.

That's what disks look like when the console is running and you rotate it from sitting flat to standing up.

Honestly, there is not much to do. If that is what happened then the scratch is deep, and the only thing to do is get a new one.

You can try to install it to your HDD but it will most likely fail.

These scratches are too deep to have any small method to fixing it like toothpaste.

If you moved the console when it was running then that's the cause and these small fixes aren't going to fix it.

Sorry man :/

My original Xbox did that to a bunch of my games before it finally went and I had to send it back to MS, do the same for your 360, might be time to send it in and let them fix the drive.

Yeah, I had a copy of the orange box and I tilted the Xbox 360 from horizontal to vertical while it was running the game, and it made an awful noise. When I opened the DVD tray it did the same thing to your L4D disk. doh!