Originally Posted by rkomar
That symbol in the top right of your screen looks like it might be the control lock symbol (a key inside a box). If so, then it means you probably set the lock by mistake, and most of the buttons won't work now. Just tap the power switch to enable and disable the lock.
I agree with rkomar that your device is locked, although the menu with the last open books etc should not show. Have you tried to reset the device (by depressing the reset button at the bottom of the device, next to the USB port)?