The power switch is actually a "software" switch. It's not like a light switch in that it doesn't actually provide or deprive the camera of power... only removing the battery will deprive the camera of power. This is why it doesn't actually just go dark when you switch off power... instead you'll usually see it perform a sensor clean cycle and then power down. The camera is technically in a kind of "sleep" mode monitoring the switch. If a Canon camera ever seems to lock up and ignore all input, the battery should be temporarily removed to "reboot" the camera.
There is no "setting" in a camera that can configure it to ignore buttons. Although... if the camera is in an automatic mode, some input is deliberately ignored to make it easier to use the camera. Put the mode dial in a "creative zone" mode (P, Tv, Av, or M) and all buttons and menus should be accessible.
You might try connecting the camera via USB cable to your computer and then attempt to control the camera via the Canon EOS Utility to see if that works. If the camera responds fine to the computer control, then likely the camera's main board and circuits are working fine and it may just be a wiring problem -- as if some cable simply isn't getting a very solid connection. If this is a wiring issue, you are probably better off sending it in for service.
Tim Campbell
5D III, 5D IV, 60Da
