Using the arrow buttons, cycle to the tab with the wrench with four dots above it. You should see a "Clear Settings" option. Use the arrow keys to highlight this option, and select it using the SET button. Next select the "Clear all camera settings" option. Select "OK", and give the camera a few seconds to reset.
I use it for video and pictures but not all the time. I recently have bought a speedlight Yongnuo 560 III. When I was taking pictures with that speedlight on it, the camera started to act strangely, changing some settings itself. From that night, the MENU button, pictures preview and other external buttons are not functioning.
6ANP.