Dissapearing panel, help!

I've pressed something or activated some hotkey combination I'm unaware of, and I need help. My layout panel - the stuff at the top and bottom of the normal screen - has vanished, and I've lost it. When I bring up any of the other screens through hotkeys or something they return, but when I close the inner screens (log, inventory, etcetera) they dissapear.

What did I do, and how do I un-do it? I've tried pressing every letter on my keyboard, and got nowhere.