Cam - there's also a mode in the Denon 3808 (Pure Direct) that when turned on totally disables the GUI. I accidentally turned this on one time and it took me quite a while to figure out what I'd done. The only way to turn it off (that I remember) is to manually press the button third from the left in the top row (see picture below).
This toggles Pure Direct On/Off, and at the same time will restore the GUI. Since Pure Direct also disables Audyssey, it sounds to me like this might be what happened... hoping so at least, b/c if not this, it might be more serious.
