I am sure you will have done this (but it did take me a few days to work out when I got my first 166).
One of the two round knobs on either side of the unit is also an on/off button - I can't remember which as I always have the unit switched on. You have to push in the button to switch the unit on or off.
Sorry if you already know this.
If you have tried everything (including the 'Dark' button) then it is possible that the backlight or the backlight inverter for the LCD screen has blown.
Unfortunately the backlight is a U shaped one and they are not easy to find.
Try this. Wait until it's dark outside and start the car normally. Get a torch or use the light from a mobile phone and shine it just above the screen. If you can faintly see the display then it is more than likely the backlight problem.
Like I say, only after you have tried everything else, might this be the cause.