How do I change my display settings if I can't see my screen? I have a macbook in clamshell mode that's plugged into an HDTV for display output.  Yesterday, I changed my display settings via the System Preferences.  However, I inadvertently picked a setting that is not supported by my screen, and now I can't figure out how to undo the change.  The screen is blank.

I can open my laptop and go to dual-screen mode and change the settings there, but as soon as I switch back to clamshell mode the system picks the broken display settings again.
Try to to go /Library/Preferences/ and search for com.apple.windowserver.plist. It's the file that gets updated when you modify the display in System Preferences (I just tried).

According to this Apple KB, it's safe to delete it. So move it on your desktop and try to reboot.