Step One: Knowing Where You Went Wrong
There is a wide variety of things you could've messed up on. The process is one that needs to followed EXACTLY or you'll end up screwing up your JTAG. What I did was a careless mistake. When you open flash 360, you are supposed to choose "Write updflash.bin to NAND" then "Raw flash without keeping current KV/config" then "ignore and continue" and "start". So, if you do it correctly the button sequence is:

Code:
A -> B -> Start -> Start -> A

or, for some JTAGs it will be:
Code:
A -> B -> Start -> A

when I was carelessly flashing my KV, I used:
Code:
A -> A -> Start -> A
this causes your xbox to have an improperly flashed KeyVault. Almost anything you do different than what you're supposed to will have the same kind of effect.

Step Two: Knowing Your JTAG's Status
This is where you'll either be very happy or very sad. We are going to find out if your JTAG can still boot into Xellous, or "Xell". As you should probably know, to do this, all you do is hit the eject button on your JTAG console. No need to hit the power button, JUST the eject button. Then turn to your T.V screen. If a blue screen with white text appears and starts doing things you dont understand, you're JTAG can boot into Xell. If it starts up like normal and you see the XBOX360 logo and appear at the dashboard, you're JTAG can't boot into Xell and you might need to spend some money. Make sure to check the space in front of the disk drive. If there is anything blocking it, Xell will not boot. Also, if you appear at the dashboard, try unplugging your xbox, plugging it back in, and hitting the eject button again. If it still doesn't work, skip to step four.

Step Three: Fixing Your JTAG (For Consoles That Can Boot Into Xell)
If you're at this step, you're lucky. Because you're Xell still works. All you need to do is plug your flash drive with the updflash.bin (the file you received after patching your KV) loaded onto it into your console. Once the flash drive has been plugged in, make sure your xbox is off, and hit the eject button to boot into Xell. It should say "searching for USB" and then shorty after it should find your USB and begin writing the NAND from the flash drive to your Xbox. If it doesn't, don't worry! We are going to do it again anyway. Wait until the text has completely stopped moving, so that your screen is completely idle as if it were a picture. Then, unplug your console. Again, no need to turn it off first, just unplug it. Wait 5-10 seconds, and plug it back in. Then, hit the eject button again. Xell should boot again, and the text should start moving. Once again, wait until the text has completely stopped, and unplug the console one last time. Wait 5-10 seconds, plug it back in, and turn on the console via the power button like normal. You're NAND is now completely recovered and you can host lobbies or go online and do whatever you do again. Try loading XeXMenu and starting MW2 before you sign in, and if it says you are banned, open Flash360.xex and dump the nand onto the flash drive and repeat the KV flashing process using 360FlashTool. Then, write the KV to your NAND like you normally would, but this time, DO IT THE RIGHT WAY! If this dosen't work, and you are using a shared KeyVault, it has been flagged and you must either ask for a refund or buy a new KeyVault. But, on the bright side, you've fixed your JTAG!

Step Four: Fixing Your JTAG (For Consoles That Can NOT Boot Into Xell)
If you made it to this step, you're unlucky. You have a couple options. The first, if you don't know how to take apart a console, aren't experienced with computers, or can't solder (in some cases), is just finding someone to do it for you. This generally costs anywhere from $30-50. I can do it for you for $30. Message me on AIM if you need to have it done. My AIM is JmKHaMeR. If you are experienced with computers, and know how to take apart a console, but can't solder, you may be in luck. Take it apart and look inside to see if there is an lpt connector soldered into it. If there is, find a tutorial on how to JTAG an xbox, and it will show you how to write your NAND to the JTAG with Nandpro. If it doesn't, you need to know how to solder.

That wraps up this tutorial guys. I put a lot of time into this, and the least I can ask for is a +Rep / Thank For The Post! If anyone wants a tutorial on flashing a KV (unbanning your JTAG) or fixing a bad KV flash with an lpt port, comment on this and ill add it to here. Until next time