Please verify the touchscreen. If the it registers permanent (ghost) input, most button input is ignored.
To make sure nothing is wrong with the buttons verify their function in GodMode9.
Gently clean the touch panel; make sure nothing is stuck between the frame and the panel (edges). A protective foil can cause this error if not applied perfectly. In the worst case, read: cleaning didn't help -- disconnect the touch panel from the motherboard to see if all buttons go back to normal again.
The touch panel is cheap though the replacement procedure isn't 100% easy. Refer to ifixit.com for instructions.
Good luck and have fun!