How Do I Fix My Xbox 360's High Definition Fixed?

I can't get my Xbox 360's HD to work. I've got the right cords and everything. I've done this many times before. The problem is that when go you from My Xbox to System Settings to Visual, it allows you to set your visual settings. (420p, 1080p, etc.) I clicked the wrong one, and my screen went black. I pressed a few buttons without realizing what I had done.. I had set that setting as my default setting! Now my Xbox 360 only works with the regular TV input cables. Whenever I use HD anymore my screen is always black. How can I fix this?

Suggestion:

I don't have my system in front of me at this second, but I'm positive there's a restore defaults setting in the system menu. That is what I would recommend as it will overwrite anything that you had inputted.