If the screen is dark suddenly on Kodi it's possible that a Kodi 'help' pop-up has appeared (like on first run). This appears as an overlay with the background dimmed. To get rid of this either use a mouse or remote mouse function to click done or similar prompt.
Best settings are subjective, but to access image settings press remote menu button whilst playing a video. Then 'Advanced Options' - 'PQ Settings'.
From here you can set brightness/contrast/hue/saturation (Display Mode) and Imprex settings (noise reduction/sharpness etc).